Applications of nano-enabled large area macroelectronic substrates incorporating nanowires and nanowire composites

ABSTRACT

Macroelectronic substrate materials incorporating nanowires are described. These are used to provide underlying electronic elements (e.g., transistors and the like) for a variety of different applications. Methods for making the macroelectronic substrate materials are disclosed. One application is for transmission an reception of RF signals in small, lightweight sensors. Such sensors can be configured in a distributed sensor network to provide security monitoring. Furthermore, a method and apparatus for a radio frequency identification (RFID) tag is described. The RFID tag includes an antenna and a beam-steering array. The beam-steering array includes a plurality of tunable elements. A method and apparatus for an acoustic cancellation device and for an adjustable phase shifter that are enabled by nanowires are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/674,071, filed Sep. 30, 2003, which claims the benefit of U.S.Provisional Application Nos. 60/414,323, filed Sep. 30, 2002; 60/445,421filed Feb. 5, 2003; 60/468,276, filed May 7, 2003; 60/474,065, filed May29, 2003; and 60/493,005, filed Aug. 7, 2003, each of which isincorporated herein in its entirety by reference.

BACKGROUND OF THE PRESENT INVENTION

1. Field of the Present Invention

The present invention relates to semiconductor devices, and moreparticularly, to the use of thin films of nanowires in semiconductordevices for various applications.

2. Background Art

An interest exists in industry in developing low cost electronics, andin particular, in developing low cost, large area macroelectronicdevices. Large-area macroelectronics is defined as the implementation ofactive and sensory electronic components over a large surface area.Here, a large area is not used to fit all of the electronic components,but rather because such systems must be physically large to realizeimproved performance and the active components of such systems must bedistributed over the large area to realize a useful functionality. Theincorporation of active devices over a large common substrate is drivenby system performance, reliability, and cost factors, not necessarily byindividual component performance. Such large area macroelectronicdevices could revolutionize a variety of technology areas, ranging fromcivilian to military applications. Example applications for such devicesinclude driving circuitry for active matrix liquid crystal displays(LCDs) and other types of matrix displays, smart libraries, creditcards, radio-frequency identification (RFID) tags for smart price andinventory tags, security screening/surveillance or highway trafficmonitoring systems, large area sensor arrays, and the like.

Current approaches involve using amorphous silicon or polysilicon as thebase materials for thin-film transistors (TFTs). Organic semiconductorsare emerging as an alternative. However, amorphous silicon and organicsemiconductors have performance limitations. For example, they exhibitlow carrier mobility, typically about 1 cm²/V·s (centimeter squared pervolt second) or less. Polysilicon has showed improved performance, butrequires relatively expensive processes, such as laser inducedannealing, and is incompatible with low temperature substrates, such ascheap glass and plastics.

Unfortunately, traditional electronic materials are characterized by aroughly inverse relationship between electronic performance (determinedprimarily by carrier mobility, μ) and available substrate size. FIG. 1is a plot that schematically illustrates materials performance(mobility) vs. available substrate size for different semiconductormaterials. Traditional materials either have high performance but smallsubstrate sizes (e.g., GaAs), or larger sizes with low performance(e.g., amorphous silicon or organics). Current electronic materials canonly access the most primitive large-area macroelectronics applications.This leaves a tremendous void in materials characteristics, which hasprevented the development of the highest-value macroelectronicapplications, such as wearable communications and electronics,distributed sensor networks, and radio frequency (RF) beam-steeringsystems, to name a few.

For instance, to realize a beam-steering reflector for use within aspace-fed antenna system, the steering-element circuits would have to bedistributed across the entire reflector, each with extreme performancerequirements typically associated with high-mobility InAs substrates.InAs wafers, however, are currently limited to a maximum of 3–4 inches(8–10 cm) in diameter and are extremely brittle, making theminappropriate for use in such large-area distributed electroniccircuits. As such, the only methods currently available for fabricatingsuch large-area circuits are to wire-bond or solder discrete transistorsand components on the large-area active reflector, a costly andfailure-prone alternative with inherent performance and efficiencylimitations. Today, even military applications of such arrays arelimited to such examples as solid communications arrays on Navydestroyers; they cannot be implemented into mobile, let aloneman-portable, communications systems.

Accordingly, what is needed are higher performance conductive orsemiconductive materials and devices, electronic substrate materials,and methods and systems for producing lower-cost, high performanceelectronic devices and components. Preferably, such materials would bereadily available, cost effectively manufactured, and enjoy otheradvantages with regards to weight, flexibility, and the like.

Many applications can benefit from such higher performance conductivesemiconductive materials, including acoustic cancellation and RFidentification (RFID) tag/reader applications. In RFID tag applications,a device known as a “tag” may be affixed to items or objects that are tobe monitored. The presence of the tag, and therefore the presence of theitem to which the tag is affixed, may be checked and monitored bydevices known as “readers.” A reader may monitor the existence andlocation of the items having tags affixed thereto through wirelessinterrogations. Typically, each tag has a unique identification numberthat the reader uses to identify the particular tag and item.

A limiting factor in the area of RFID tag tracking systems is the costof the tags. Further limiting factors include the distance between thereader and tags, and the orientation of tag antennas relative to thereader antenna. If a tag antenna is not oriented properly relative tothe reader antenna, the tag must be close to the reader to be detected.

These limiting factors are critical when attempting to read multipletags affixed to items within a container that is in transit from onelocation to another. For example, a shipping truck may pass through acheckpoint at 60 mph. If the truck is transporting a large number oftagged items, such as tens or hundreds of thousands of items, the truckmust be within a reader's range long enough to detect all of the tags.If each item within the container on the truck is randomly oriented,causing a maximum read distance for the container to be low, the readermay only have a few seconds to read all of the tags. Current tag andreader technology is not capable of reading such a large number of itemsin a few seconds.

Thus, what is also needed are methods and systems for increasing a readrate for tags, for increasing a distance over which the tags may beread, and for lower cost tags.

In acoustic cancellation applications, an attempt is made to cancel orreduce specific frequencies of sound, such as the cancellation orreduction of noise. For example, in some instances it may be desirableto partially or completely cancel the sound emanating from objects suchas a car, a bus, or even an airplane. In military applications, it maybe desirable to partially or completely cancel sounds from objects suchas a tank or submarine. Some conventional headphones incorporatetechnology that monitors noise around the headphones, and transmits apattern of acoustic waves in an attempt to substantially cancel theoutside noise. The transmitted pattern of acoustic waves is transmittedwith an opposite phase to that of the noise. This transmitted patternattempts to silence the noise, making it easier to hear what is beingplayed through the headphones. However, such technology is limited torelatively small size devices, such as headphones, and cannot be appliedto the large objects mentioned above.

Thus, what is further needed are methods and systems for performingacoustic cancellation that effectively operate to cancel sounds and/ornoise over any size area, including large areas.

BRIEF SUMMARY OF THE PRESENT INVENTION

The present invention is directed to a paradigm shift in materialstechnology; namely, applying nanomaterials to the field ofmacroelectronics. The result is a revolutionary new high-performancelarge-area macroelectronics technology on a variety of substrates, forexample plastics, that: (1) outperforms single-crystal silicon wafers(μ>5,000 cm²/V·s and on/off current ratio I_(on)/I_(off)>10⁷ with athreshold potential V_(on)<1 V); (2) can be applied to extremely largesurface areas (A>10 m²); (3) has the flexibility of polymer electronics(radius of curvature r<1 mm); and/or (4) can be processed and patternedusing traditional large-area semiconductor processing techniques likethose used to process amorphous silicon, as well as advancedlithographic techniques such as roll-to-roll screen-printing.

This technology combines the extraordinary conductive properties of anew type of nanomaterial (inorganic semiconductor nanowires) withlarge-area macroelectronics by producing dense films of preferentiallyoriented nanowires that span the gap between each source and drainelectrode within a device. The result is an electronic material for usein large-area macroelectronics with mobility (μ) and current capacity(J) equal to or greater than that of single-crystal silicon. Byincorporating alternative nanowire materials such as InAs or GaAs, evenhigher-performance substrates can be realized. This new-materialtechnology (referred to herein as dense, inorganic and oriented nanowire(DION) thin-film technology and mixed-composition DION thin-filmtechnology) can fill the void in large-area electronic materials (asshown at FIG. 1) to enable the full vision of macroelectronics incommercial, military, and security application. For example, FIG. 2shows several potential applications of true high-performancemacroelectronics.

The present invention is also directed to methods, systems, andapparatuses for an electronic substrate having one or more semiconductordevices formed thereon is described. A thin film of semiconductornanowires is formed on a substrate. The thin film of nanowires is formedto have a sufficient density of nanowires to achieve an operationalcurrent level. A plurality of semiconductor regions are defined in thethin film of nanowires. Contacts are formed at the semiconductor deviceregions to thereby provide electrical connectivity to the plurality ofsemiconductor devices.

In aspects of the present invention, semiconductor devices incorporatingthin films of nanowires are used in many applications.

In an aspect of the present invention, a method and apparatus for anadjustable phase shifter is described. A conductor line on a substrateincludes a first conductive segment and a second conductive segment. Athin film of nanowires is formed on the substrate in electrical contactwith the first conductive segment and the second conductive segment. Aplurality of gate contacts are in electrical contact with the thin filmof nanowires, and are positioned between the first conductive segmentand the second conductive segment. A phase of an electrical signaltransmitted through the conductor line is adjusted by changing a voltageapplied to at least one gate contact of the plurality of gate contacts.

In another aspect of the present invention, a method and apparatus for aradio frequency identification (RFID) tag is described. The RFID tagincludes an antenna and a beam-steering array. The beam-steering arrayincludes a plurality of tunable elements. Each tunable element includesa plurality of phase-adjustment components and a switch corresponding toeach phase-adjustment component. The switch includes a transistor formedby a film of nanowires. The switch enables the correspondingphase-adjustment component to change a phase of the tunable element.

An electromagnetic (EM) signal transmitted by the antenna is redirectedby the beam-steering array. In one aspect, the beam-steering arrayfocuses the EM signal. In another aspect, the beam-steering arrayspreads the EM signal. In still another aspect, the beam-steering arraychanges a direction of the EM signal.

In aspects of the present invention, each phase-adjustment elementcomprises an inductor, such as a micro-strip inductor, or a capacitor.

In an aspect of the present invention, the beam-steering array is abeam-steering reflector. The tunable elements are tunable cells that areco-planar. Each tunable cell includes a resonant structure. Each switchenables the electrical coupling of the corresponding phase adjustmentcomponent to the resonant structure to change a phase of the tunablecell.

In another aspect of the present invention, a method and apparatus for aRFID tag is described. The RFID tag includes a beam-steering array. Thebeam-steering array includes a plurality of tunable antenna elements.Each tunable antenna element includes a plurality of phase-adjustmentcomponents and a switch corresponding to each phase-adjustmentcomponent. The switch includes a transistor formed by a thin film ofnanowires in electrical contact with source and drain contacts. Theswitch enables the corresponding phase-adjustment component to change aphase of the tunable antenna element. An EM signal transmitted by thebeam-steering array is directed by controlling the phase of each of theplurality of tunable antenna elements.

In an aspect of the present invention, the tunable elements are tunabletransmission line segments. The switch shorts the transmission linesegment to change a length of the transmission line segment to change aphase of the respective tunable antenna element.

In further aspects of the present invention, a method and apparatus fora RFID reader is described. The RFID reader includes a beam-steeringarray, such as the beam-steering arrays mentioned above for RFID tags.

In another aspect of the present invention, a method and apparatus forproviding acoustic cancellation is described. An acoustic cancellationdevice includes a substrate and a plurality of acoustic cancellationcells formed in an array on a surface of the substrate. Each acousticcancellation cell of the plurality of acoustic cancellation cellsincludes an acoustic antenna, a processor, a transistor, and anactuator. The acoustic antenna receives a first acoustic signal. Theprocessor processes the received first acoustic signal, and generates acorresponding cancellation control signal. The transistor includes athin film of nanowires. The thin film of nanowires is in electricalcontact with a drain contact and a source contact of the transistor. Agate contact of the transistor is coupled to the cancellation controlsignal. The actuator is coupled to the transistor. The transistor causesthe actuator to output a second acoustic signal according to thecancellation control signal. The second acoustic signal substantiallycancels the first acoustic signal.

In an aspect of the present invention, the second acoustic signal has asubstantially opposite phase compared to the first acoustic signal.

In one aspect, the actuator includes an audio speaker.

In another aspect, the actuator includes a thin film of piezoelectricnanowires. The transistor allows a current to flow through the thin filmof piezoelectric nanowires to generate the second acoustic signal.

In an alternative aspect, the actuator and transistor are combined. Thetransistor includes a thin film of piezoelectric nanowires. When currentflows through the thin film of piezoelectric nanowires of thetransistor, the second acoustic signal is generated.

These and other objects, advantages and features will become readilyapparent in view of the following detailed description of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of thepresent invention and to enable a person skilled in the pertinent art tomake and use the present invention.

FIG. 1 is a plot that schematically illustrates materials performance(mobility) vs. available substrate size for different semiconductormaterials (as indicated in the blue areas).

FIG. 2 shows several potential applications of true high-performancemacroelectronics.

FIG. 3 illustrates semiconductor nanowire materials and quality.

FIG. 4 illustrates a single nanowire FET.

FIG. 5 schematically illustrates the length scale of order.

FIG. 6 schematically illustrates the length scales of nanowires.

FIG. 7 illustrates a semiconductor/dielectric core-shell structure.

FIG. 8 illustrates a transmission electron microscope (TEM) and energydispersive X-ray analysis (EDX) images of the cross section andcomposition of a multi-shell nanowire with a germanium core, a siliconinner shell and a silica outer shell.

FIG. 9 shows a dark-field optical micrograph (OM) of a nanowire withbend r<10 μm.

FIG. 10 shows a view of a portion of a thin film of nanowires, accordingto an example embodiment of the present invention.

FIG. 11 shows a semiconductor device that includes a thin film ofnanowires, according to an example embodiment of the present invention.

FIGS. 12A–12D shows nanowires doped according to various exampleembodiments of the present invention.

FIGS. 13A and 13B show examples of a semiconductor device, dopedaccording to example doping embodiments of the present invention.

FIG. 14 shows an overview of a multifunctional mixed nanowire thin film.

FIGS. 15A, 15B, and 15C show preliminary results for p-doped siliconnanowire thin-film transistors (μ˜100 and on/off ˜10⁴).

FIG. 16 shows a schematic of a DION TFT.

FIG. 17 shows schematics of a multi-core-shell nanowire comprising asemiconductor core, a passivation shell, an insulating gate dielectricshell, and a conducting gate shell.

FIG. 18 shows a process flow of universal electronic substrate formacroelectronic circuit fabrication.

FIG. 19 schematically shows etching of shell materials in amulti-core-shell nanowire to expose the core material for source draincontact in a nanowire TFT.

FIG. 20 shows several possible configurations of a mixed-compositionDION film.

FIG. 21 schematically illustrates an example of using amixed-composition DION thin-film to form a CMOS circuit.

FIG. 22 shows a schematic of a generic reactor for fabrication ofsemiconductor nanowires.

FIGS. 23A and 23B illustrate a standard nanowire FET testing platform.

FIG. 24 schematically illustrates a DION thin-film transistor using aglobal back-gate.

FIG. 25 shows a schematic view of locally gated nanowire thin filmtransistors.

FIGS. 26A and 26B are, respectively, a schematic and a layoutillustration of a static CMOS two-input NAND gate.

FIG. 27 schematically illustrates a system for roll-to-roll compatibleflow-based DION film deposition.

FIG. 28 schematically illustrates the concept of a distributed sensornetwork of the present invention.

FIG. 29 shows a schematic illustration of an RFID/Sensor tag system.

FIG. 30 illustrates a RFID tag and reader communications environment,according to an example embodiment of the present invention.

FIG. 31A shows an example RFID tag, according to an embodiment of thepresent invention.

FIG. 31B shows an example RFID reader, according to an embodiment of thepresent invention.

FIG. 32 shows an example beam-steering array operating in a transmittingmode.

FIG. 33 shows a surface of an example beam-steering array, according toan embodiment of the present invention.

FIG. 34 shows a cross-sectional view of an example, fixed-frequencyperfect magnetic conductor (PMC) structure, according to an embodimentof the present invention.

FIG. 35 shows a perspective view of a portion of the PMC structure ofFIG. 34, including a 2×2 array of cells.

FIG. 36 illustrates inductance and capacitance involved in resonance oftwo cells of an example fixed-frequency PMC structure.

FIG. 37 shows a transmission-line equivalent schematic view of the pairof cells of the PMC structure of FIG. 36.

FIG. 38 shows a cross-sectional view of a portion of a beam-steeringarray, where active phase-adjustment elements are coupled to a PMCstructure to provide discrete tunability, according to an exampleembodiment of the present invention.

FIG. 39 shows a transmission-line equivalent schematic view of the pairof cells of the PMC structure of FIG. 38.

FIG. 40 shows a plot of a reflection coefficient phase versus afrequency for a PMC structure.

FIG. 41 shows a scale drawing where three conventional inductors andcorresponding switches are mounted to a cell of a beam-steering array.

FIG. 42 shows a scale drawing of a cell of a beam-steering array thatmounts a nanowire-based phase-adjustment circuit, according to anembodiment of the present invention.

FIG. 43 shows a detailed view of an example nanowire-based transistormounted on the cell of FIG. 42, according to an embodiment of thepresent invention.

FIG. 44 illustrates that formation of a beam-steering array, accordingto an example embodiment of the present invention.

FIGS. 45 and 46 show nanowire-based phase adjustment circuits beingformed on a PMC structure, according to an example embodiment of thepresent invention.

FIG. 47 shows an example multi-antenna element beam-steering array,according to an embodiment of the present invention.

FIG. 48 shows a flowchart providing example steps for forming anadjustable phase shifter on a substrate, according to embodiments of thepresent invention.

FIG. 49 shows a conductor line formed on a substrate.

FIG. 50 shows a thin film of nanowires formed on the substrate of FIG.49, according to an embodiment of the present invention.

FIG. 51 shows a plurality of gate contacts formed in electrical contactwith the thin film of nanowires of FIG. 50 to form an adjustable phaseshifter, according to an embodiment of the present invention.

FIGS. 52A and 52B show views of an example adjustable phase shifter withaligned nanowires, according to an embodiment of the present invention.

FIG. 53 shows a plurality of thin films of nanowires formed on asubstrate to create an adjustable phase shifter, according to anembodiment of the present invention.

FIG. 54 shows a conductor line with incorporated loads to provideadditional phase delay, according to an example embodiment of thepresent invention.

FIG. 55 shows the conductor line of FIG. 54, with thin films ofnanowires formed thereon to form a plurality of nanowire-basedtransistors.

FIG. 56 shows a substrate that incorporates an array of actuators andrelated electronics for active acoustic cancellation, according to anembodiment of the present invention.

FIG. 57 shows an example application of the array of FIG. 56 to monitorand cancel noise, according to an embodiment of the present invention.

FIG. 58 shows an example detailed block diagram of a cell of the arrayof FIG. 56, according to an embodiment of the present invention.

FIG. 59 shows a 2×2 acoustic cancellation array, with each cell of thearray configured as shown in FIG. 58, according to an example embodimentof the present invention.

FIG. 60 shows an array similar to the array of FIG. 56, with each cellincorporating a nanowire-based interface circuit, according to anexample embodiment of the present invention.

FIG. 61 shows an array similar to the array of FIG. 60, which each cellfurther incorporating a nanowire-based actuator, according to an exampleembodiment of the present invention.

FIGS. 62 and 63 show nanowire-based interface circuits and actuators,according to example embodiments of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Introduction

In many ways, the electronics industry today is in the same position itwas fifty years ago. At that time, the introduction of discretetransistors provided the world with unprecedented functionality. Byintegrating multiple discrete transistors together, functionality wasfurther increased, leading to the broad availability of portableelectronics. Unfortunately, at that time, integration was done by hand,causing the electronics industry to rapidly reach a plateau, beyondwhich integrating more than a few thousand individual transistors becameprohibitive in terms of cost and yield. This phenomenon was referred toas the “Tyranny of Numbers”.

This practical limit, resulting from the need for heterogeneousintegration of multiple discrete components, brought the integratedelectronics industry to a standstill that was only resolved through theinvention of a revolutionary new concept: the silicon microcircuit. Thesilicon microcircuit allowed seamless integration of a virtuallylimitless number of electronic components into a single device using asimple linear process: a true unifying platform for integratedelectronics. It was this invention that allowed the integratedelectronics industry to grow as it has, increasing circuit density (andtherefore device functionality) in accordance with Moore's Law.

In many ways, the approaching end of Moore's Law resulting fromtransistors inevitably reaching their fundamental size limit (a fewatoms wide) will create a situation analogous to the limit reached bytransistors in the 1950s. As this fundamental circuit density (and thepractical size-limit of semiconductor wafers) limit is approached,increased functionality will only continue to be achieved through theintegration of multiple discrete components into higher-order devices.As such, the integrated circuits (ICs) of today are analogous to thetransistors of the 1950s: an important discrete functional-unitrequiring heterogeneous integration to expand its functionality.

In this sense, heterogeneous integration can involve combining multipleprocessors to perform a function, or combining processors with otherdiscrete components such as memory, sensors, radio frequency (RF)electronics, antennas, active optical components, actuators, etc. Theeffects of the limit of heterogeneous integration have not yet beentruly felt (development is still at a stage where only a small number ofcomponents must be integrated). However, eventually, as with thetransistors of the 1950s, an analogous “Tyranny of Numbers” willinevitably be reached as the number of discrete functional unitsincreases over time. Even today, heterogeneous integration of discreteICs into higher-functional devices represents one of the largest costcomponents and the primary source of device failure in manufacturedelectronic devices. Without the introduction of a unifying platform uponwhich to integrate higher-functionality with today's ICs, within thenext twenty years this will become the ultimate limitation on the growthof this world-reliant industry. No such platform currently exists.

The mixed-film macroelectronics technology of the present inventionrepresents a fundamental underpinnings of a truly unifying platform foruniversal functional device integration. The description of the presentinvention herein focuses on tailoring functionality to producehigh-performance transistors over large areas using nanowires withcompositions selected for high conduction-mobility and integrateddielectric layers. However, by incorporating alternative nanowirematerials, the same platform can be expanded to include high-performanceoptical, magnetic, ferroelectric, and piezoelectric properties as well.Building off of this fundamental platform, the present inventionincorporates multiple different functionalities (e.g., high-performanceelectronic plus active optical) onto the same substrate to impartmultiple different functionalities into the same material. Thistechnology represents a true separation of structure and function: aparadigm shift in materials technology.

At its core, the technology described here represents a fundamentalunifying materials platform capable of eventually enabling globalintegration of all different functionalities into a single device thatcan be fabricated using a standard linear process. Similar to thesilicon IC, as the platform expands, all levels offunctional-integration can be incorporated, indefinitely extending thespirit of (if not the specific definition of) Moore's Law.

The present invention is directed to a revolutionary newthin-film-on-plastic (or other useful substrate materials) technologybased on dense films of oriented inorganic semiconductor nanowire(nanowires) aligned in parallel (or substantially aligned in parallel).As used herein, the term “nanowire” generally refers to any elongatedconductive or semiconductive material that includes at least one crosssectional dimension that is less than 500 nm, and preferably, less than100 nm, and has an aspect ratio (length:width) of greater than 10,preferably, greater than 50, and more preferably, greater than 100, andeven greater than 500. For example, in an embodiment, a nanowire canhave a diameter in the 1–100 nm range, and a length in the 1–100micrometer range. In other embodiments, nanowires can have diameters andlengths in other ranges.

Examples of such nanowires include semiconductor nanowires as describedin Published International Patent Application Nos. WO 02/17362, WO02/48701, and 01/03208, carbon nanotubes, and other elongated conductiveor semiconductive structures of like dimensions. Particularly preferrednanowires include semiconductive nanowires, that are comprised ofsemiconductor material selected from, e.g., Si, Ge, Sn, Se, Te, B, C(including diamond), P, B—C, B—P(BP6), B—Si, Si—C, Si—Ge, Si—Sn andGe—Sn, SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb,InN/InP/InAs/InSb, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb,InN/InP/InAs/InSb, ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe,BeS/BeSe/BeTe/MgS/MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS,PbSe, PbTe, CuF, CuCl, CuBr, CuI, AgF, AgCl, AgBr, AgI, BeSiN2, CaCN₂,ZnGeP₂, CdSnAs₂, ZnSnSb₂, CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl,Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄, Al₂O₃, (Al, Ga, In)₂ (S, Se, Te)₃, Al₂CO,and an appropriate combination of two or more such semiconductors. Incertain aspects, the semiconductor may comprise a dopant from a groupconsisting of: a p-type dopant from Group III of the periodic table; ann-type dopant from Group V of the periodic table; a p-type dopantselected from a group consisting of: B, Al and In; an n-type dopantselected from a group consisting of: P, As and Sb; a p-type dopant fromGroup II of the periodic table; a p-type dopant selected from a groupconsisting of: Mg, Zn, Cd and Hg; a p-type dopant from Group IV of theperiodic table; a p-type dopant selected from a group consisting of: Cand Si.; or an n-type is selected from a group consisting of: Si, Ge,Sn, S, Se and Te.

In certain instances the term “nanowire” as used herein also encompasses“nanotubes”. These instances will be identified. Nanotubes can be formedin combinations/thin films of nanotubes as is described herein fornanowires, alone or in combination with nanowires, to provide theproperties and advantages described herein.

Furthermore, it is noted that thin film of nanowires of the presentinvention can be a “heterogeneous” film, which incorporatessemiconductor nanowires (and/or nanotubes, as identified), and/ornanowires (and/or nanotubes, as identified) of different compositionand/or structural characteristics. For example, a “heterogeneous film”can includes nanowires/(and/or nanotubes, as identified) with varyingdiameters and lengths, and nanowires (and/or nanotubes, as identified)that are “heterostructures” having varying characteristics.

In the context of the present invention, the substrate to whichnanowires are attached may comprise: a uniform substrate, e.g., a waferof solid material, such as silicon, glass, quartz, polymerics, etc.; alarge rigid sheet of solid materials, e.g., glass, quartz, plastics suchas polycarbonate, polystyrene, etc., or can comprise additionalelements, e.g., structural, compositional, etc. A flexible substrate,such as a roll of plastic such as polyolefins, polyamide, and others, atransparent substrate, or combinations of these features can beemployed. For example, the substrate may include other circuit orstructural elements that are part of the ultimately desired device.Particular examples of such elements include electrical circuit elementssuch as electrical contacts, other wires or conductive paths, includingnanowires or other nanoscale conducting elements, optical and/oroptoelectrical elements (e.g., lasers, light emitting diodes (LEDs),etc.), and structural elements (e.g., microcantilevers, pits, wells,posts, etc.).

By substantially “aligned” or “oriented” is meant that the longitudinalaxes of a majority of nanowires in a collection or population ofnanowires is oriented within 30 degrees of a single direction. Althoughthe majority can be considered to be a number of nanowires greater than50%, in various embodiments, 60%, 75%, 80%, 90%, or other percentage ofnanowires can be considered to be a majority that are so oriented. Incertain preferred aspects, the majority of nanowires are oriented within10 degrees of the desired direction. In additional embodiments, themajority of nanowires may be oriented within other numbers or ranges ofdegrees of the desired direction.

It should be understood that the spatial descriptions (e.g., “above”,“below”, “up”, “down”, “top”, “bottom”, etc.) made herein are forpurposes of illustration only, and that devices of the present inventioncan be spatially arranged in any orientation or manner.

Semiconductor nanowires can be synthesized off-line, and then depositedon plastic or any other substrate at low temperature. After deposition,the dense thin-film of nanowire can be further processed at lowtemperature using traditional thin-film transistor (TFT) manufacturinginfrastructure to produce high-performance macroelectronic devices onplastic with electronic performance comparable to or exceeding that ofbulk semiconductor devices of similar dimensions. In addition, thistechnology allows the fabrication of films containing nanowires of twoor more different semiconductor compositions. This allowsmulti-functional macroelectronic systems incorporating logic, RF, lightemission, light detection and more, all on a single substrate to berealized using a single linear manufacturing process (i.e., homogeneousintegration of different electronic functionalities on a singlemonolithic substrate). This breakthrough technology opens up a broadrange of new applications that had previously been too expensive todevelop. These include, for example, distributed sensor networksfabricated on plastic or other useful substrates, which carry outsensing, computation, and remote communication throughout the network.

One of the important advantages of the present invention is the abilityto fabricate high-performance large-area distributed macroelectronics atlow-resolution and moderate complexity (e.g., 1000 s of transistors percircuit) on lightweight flexible substrates at a commercially reasonablecost, e.g., less than $100/ft². Such abilities enable the development ofnext-generation sensor and security applications with increasedfunctionality, decreased size, decreased weight, and decreased powerrequirements compared to what is possible using traditionaltechnologies. Yet these abilities can be realized at a cost that makesthese new applications viable for real-world deployment.

Traditional TFT manufacturing processes for amorphous silicon are morethan adequate to achieve the device complexity at costs of $100/ft² forcurrent macroelectonic applications (e.g., TFT backplanes for liquidcrystal displays (LCDs) are manufactured for less than $30/ft²).However, the thin-film material of the present invention is not onlycompatible with these manufacturing processes (or with new, lower costprocesses), but also realizes performance of more than two orders ofmagnitude greater than amorphous silicon.

In addition, in the development of high-value applications inmacroelectronics, e.g., military, space, and defense applications, whichare space, weight, and power constrained, it is highly desirable to beable to develop technologies that are compatible with complementarymetal oxide semiconductor (CMOS) technology (e.g., for low power), andintegrate other functionalities (e.g., light emission) into a singlefilm to reduce the number of discrete components that need to beassembled to fabricate each device (decreasing cost, weight, andcomplexity, while increasing yield and functionality).

The revolutionary new thin-film technology of the present invention isbased on dense, inorganic and oriented nanowire thin-films (DIONthin-films). DION thin-films can enable high-performance large-areamacroelectronics with multiple different electronic, electro-optic andelectromechanical functionalities all on a single lightweight flexiblesubstrate that can be fabricated over large areas (>10 m²) at a cost ofless than $100/ft². An important aspect of DION thin-film technology isthat carrier conduction occurs within aligned single crystal nanowires.Hence single crystal mobility can be achieved in these macroelectronicdevices. Furthermore, by exploiting quantum effects in nanowires,devices realized with the present invention can enjoy performancesexceeding that of conventional bulk single crystals. Another importantaspect of DION thin-film technology is that the high temperature stepsrequired to synthesize single-crystal nanowires, which include anintrinsic high-quality gate-dielectric shell and a conformalgate-electrode around each individual nanowire, can be carried outoff-line in reactors before the nanowires come into contact with thesubstrate. As a result, all processing steps that occur in the presenceof the substrate material can be done at low temperature (<100° C.),allowing the use of many different substrate materials (e.g., flexible,low-T_(g) (temperature of glass transition) plastics). DION thin-filmtechnology can allow the fabrication of large-area flexiblemacroelectronics with performance exceeding that of single-crystalsilicon. Additionally, DION thin-film technology can allow thefabrication of mixed-functionality monolithic electronics that cannot befabricated using any current technology (e.g., combining the mobility ofInAs, the CMOS performance of Si, and the light emission of GaN all on asingle substrate). The result is a large-area macroelectronicstechnology that can outperform existing technologies, while producinglightweight, flexible electronics over large areas at low cost.

While this technology enables numerous possible combinations offunctionality into a single film (e.g., electronic, optical, magnetic,ferroelectric, piezoelectric, etc), the following discussion focuses onhigh-performance electronics. In particular, the discussion focuses onthe integration of high-performance n- and p-channel silicon nanowiresfor CMOS functionality (for low-power devices), and extreme-mobilityIII–V materials such as InAs and InP for RF processing, all on a singlemonolithic plastic substrate. Of course, it will be recognized by one ofordinary skill that a large number of different uses, applications, andsystems can be enabled by the technology described herein.

The mixed-composition DION thin-film technology can enable thedevelopment of a variety of unique applications ranging from RFcommunications, to sensor arrays, to X-ray imagers, to flexible displaysand electronics, and more. In addition, it can establish a foundationfor a variety of high-value commercial technologies, includinglightweight disposable or flexible displays with driver-electronicsprinted onto a single substrate, “penny”-RFID tags for universalRF-barcoding, integrated sensor networks for industrial monitoring andsecurity applications, and phased-array antennas for wirelesscommunications. The DION technology can revolutionize both the militaryand commercial world of large-area electronics.

FIG. 3 illustrates semiconductor nanowire materials and quality. Theupper-right image shows nanowires at different magnifications as seenthrough an electron microscope showing the quality and uniformity ofthese materials. The lower-right image shows vials of different NWmaterials suspended in solution.

One of the truly unique properties of these materials is that theirelectronic and conductive properties can be exactly defined, includingcrystal structure, doping density, mobility, bandgap, etc. In addition,when synthesized, every nanowire is the same as every other nanowirewithin a batch (and between batches). This feature stands in strikingcontrast to other common nanomaterials, such as carbon nanotubes, whereevery nanotube within a batch is different from every other one, withelectronic properties ranging from metallic to semiconducting tosemimetallic. The ability to produce large volumes of nanowires withevery nanowire having the same electronic properties facilitates the useof DION thin-film technology.

Single-nanowire electronic circuits have been fabricated including p-ndiodes and field-effect transistors (FETs) (see, e.g., Y. Huang, X.Duan, Y. Cui, and C. M. Lieber “Gallium Nitride Nanowire Nanodevices,”Nano Letters, 2, 101–104 (2002); Y. Cui, Z. Zhong, D. Wang, W. Wang, C.M. Lieber, Nano Lett. 3, 149 (2003); and X. Duan, Y. Huang, Y. Cui, J.Wang and C. M. Lieber, Nature 409, 66 (2001)). FIG. 4 illustrates asingle nanowire FET. The left portion shows a scanning electronmicrograph (SEM) image and a schematic drawing of a single nanowire FET.The middle and right portion shows the field-effect performance of n-and p-doped InP and Si single-nanowire FETs, respectively. Due to thehigh quality of these materials (true single crystals) combined withsuppressed scattering probabilities arising from quantum-confinedstates, these inorganic semiconductor nanomaterials have been shown tohave mobilities that exceed their bulk materials over distances greaterthan 100 μm. For example, field-effect mobilities of μ=1,500 cm²/V·shave been demonstrated for Si nanowires and μ>4,000 cm²/V·s for InPnanowires. These values are comparable or superior to theirsingle-crystal counterparts with similar doping concentrations. Thesemobilities are believed to represent only a lower limit for thesematerials. In fact, theoretical calculations have predicted μ=3×10⁸cm²/V·s for selectively doped GaAs nanowires.

The performance characteristics of these single-nanowire devices areextremely encouraging with respect to nanoelectronic applications. Manyacademic groups are currently pursuing the development of nanoelectroniccircuits to make electronics faster and smaller. However, the totalamount of current that can be carried by a single nanowire is verysmall, which limits the ability of single-wire devices tonanoelectronics applications such as nano-logic and nano-memory.Fortunately, the present invention provides a complete paradigm shift:using the same nanomaterials to make electronics faster and larger, butusing the advances made in the work to date performed on such singlewire devices.

Performance in electronic materials is characterized by the length scale(l) over which the material displays “order” compared to the lengthscale of the device in which it is used. FIG. 5 schematicallyillustrates the length scale of order. Mobility in a transistor isrelated to the number of traps and grain boundaries that a charge mustpass as it travels between the source and the drain electrodes. On thelength scale of macroelectronics (10–40 μm), amorphous andpolycrystalline silicon have many such defects, resulting in poormobilities. In contrast, single-crystal silicon can easily span theentire distance between electrodes without crossing a grain boundary,providing a device with performance that is limited only by theintrinsic mobilities of the material. For example, a single-crystalsilicon wafer is “ordered” on l>1 cm and has μ>1,000 cm²/V·s for anydevice that is smaller than this length scale. In contrast,polycrystalline silicon is “ordered” on only 100≦l≦500 nm (approximatelythe grain size) and has 10≦μ≦50 cm²/V·s, while amorphous silicon, whichis “ordered” on only l<10 nm, has μ≈1 cm²/V·s. Thus, the mobility ofamorphous silicon is one thousand times lower than that ofsingle-crystal silicon. Similarly, even the best organic semiconductorsare ten to one hundred times still lower in performance.

Inorganic semiconductor nanowires are a unique material with respect to“length scale of order.” In one dimension (across the diameter of thenanowire), they are “ordered” over only a few nanometers. Along theother dimension, however, they display order over 100 μm. This is why,on the length scale of macroelectronics, single-nanowire electronicdevices display such high mobilities; on this length scale, they actlike a single-crystal semiconductor. Unfortunately, while the mobilityin a single-nanowire TFT may be high, the total current that can travelthrough a single nanowire is still extremely low.

The present invention leverages the extreme asymmetry in the “lengthscale of order” of inorganic semiconductor nanowires to produce a newmacroelectronic substrate material that realizes a substantialimprovement in performance. FIG. 6 schematically illustrates the lengthscales of nanowires. The schematic depiction of the length scales ofnanowires and of macroelectronics shows how these materials can formuniform high-performance materials on the length scale ofmacroelectronics. On the length scale of macroelectronic electrodes(l≈10–20 μm), thousands of nanowires can be placed side-by-side(parallel to each other) in a pseudo-close-packed film across the spanof a single electrode. Each of these nanowires is substantially longerthan the distance between electrodes (100 μm vs. 20 μm). Therefore,virtually all of these nanowires will span the entire semiconductingchannel to produce thousands of high-mobility conductive channels. Byrandomly staggering the starting point for each nanowire, the existenceof “seams” in the material (the equivalent of a grain boundary) can belargely eliminated. In this manner, any individual nanowire that doesnot span the gap will be statistically averaged out over thesubstantially larger number of nanowires that do.

It is important to note that, since there is no commercially viable wayto purify or filter carbon nanotube samples to separate metallic fromsemiconducting nanowires, carbon nanotubes cannot practically be usedfor form DION films for this type of application. The presence of even asingle metallic nanowire within a device would substantially diminishtransistor function. Similarly, since performance of each individualdevice depends on the average performance of the nanowires within thatdevice, uniformity of materials characteristics is important. This hasyet to be achieved in carbon nanotubes. Inorganic semiconductornanowires enable this technology.

In addition to high mobilities, such dense nanowire films formacroelectronics have several other critical advantages. By way ofexample, such materials can have extremely thin, pinhole freegate-dielectric shells conformally surrounding each individual nanowirefor low-voltage, low-power operation. Preliminary results fromsingle-nanowire FET devices indicated that it is possible to produce anextremely thin, pinhole-free oxide shell around each nanowire duringsynthesis. FIG. 7 illustrates a semiconductor/dielectric core-shellstructure. A schematic view of a nanowire core-shell structure with acrystalline core and integrated dielectric shell is shown. Theintegrated thin dielectric shell on the nanowire surface not onlyeliminates the high temperature process required to deposit thedielectric on substrate, but also affords a low threshold voltage.

Because the nanowires and shells are formed off of the substrate, theycan be processed at high-temperature (>500° C.) to ensure the highestquality dielectric without affecting the choice of substrate material.The ability to form a high-quality dielectric is strongly dependent onthe semiconductor used, with Si being the best example. This oxidelayer, which can be as thin as 1–2 nm, can be used to replace theextrinsic gate-oxide in a FET, thus reducing the number of processingsteps required and dramatically reducing the voltage required to turnthe transistor on or off. Single-nanowire devices have been activatedwith less than 1 V of gate potential. Thus, very low power,high-mobility DION macroelectronics can also be produced. This is insharp contrast to other thin-film low-temperature semiconductors, suchas laser-annealed p-Si, which require thick dielectric layers formed atlow temperatures in the presence of the substrate, resulting in limitedsubstrate compatibility and high-power operation.

These materials also provide advantages in terms of uniformity of deviceperformance across the thin-film, leading to low-cost device fabricationand extremely low power operation. One of the major limitations to otherhigh-performance thin-film technologies, such as large-grain p-Si, isdevice-to-device uniformity due to the presence of grain-boundariesbetween source and drain electrodes. When grain boundaries are verysmall relative to the size of the transistor channel, the transistorsuffers from poor mobility, but benefits from statistical averaging ofthe number of grains per device. That is to say, the number of grainsper device is so large that each device has essentially the same numberof grains and the same types of grains, so they all perform the same.The result is that each device performs the same as every other device;this is the case for amorphous silicon. Unfortunately, as the grain sizeincreases, increasing overall mobility, the number of grains per devicebegins to decrease so that each device across a thin-film samples astatistically different numbers and type of grains. The result is that,as mobility increases with p-Si, uniformity in device characteristicsdecreases. This is particularly problematic in the variation ofthreshold voltage from device to device in high-mobility p-Si. Tocompensate for this intrinsic problem, it is necessary to either: (1)adjust for the non-uniformity by increasing the complexity of thecircuit or (2) adjust for the non-uniformity by increasing the appliedgate-voltage to all devices to ensure that all devices turn on under thesame applied voltage. The fist option increases the number of mask stepsfor lithography (approximately twice as many mask steps) and as a resultcan dramatically increase the cost of devices. The second option cansubstantially increase the power consumption of the device. Since two ofthe primary requirements for certain high performance macroelectronicssystems are cost and power consumption, neither of these solutions isacceptable in those applications.

The situation is quite different for DION thin-films. As a result of theextreme aspect ratio and alignment of the nanowires within a DION film,TFTs fabricated from these films are extremely uniform across the film.In one dimension, the nanowires can be more than ten times longer thanthe TFT channel-length so that virtually all nanowires span the entirechannel (i.e., no grain boundaries in the direction of conduction). Inthe second dimension (the non-conducting direction), the nanowires areup to one thousand times smaller than the channel width, so that eachtransistor can easily contain hundreds to thousands of individual“grains”. As a result, each transistor sees no grain-boundaries in theconducting direction and a true ensemble average in the non-conductingdirection. This not only produces individual high-performance devices,but also eliminates differences from device to device throughlarge-number statistical averaging. Thus, TFTs fabricated from DIONfilms enjoy far greater uniformity than is currently available with p-Sior organic electronics.

Additionally, these materials provide advantages relative to theirapplicability to many different nanowire materials allowing DIONthin-film devices to be fabricated containing many different functionaldevices, each with the performance of single-crystal semiconductors:DION thin-films are not limited to silicon nanowires. The samearchitecture can be used to form large-area macroelectronics substratesfrom even higher-performance materials, such as GaAs or InAs, or tofabricate nanowires with unique optical or mechanical properties, suchas electroluminescent nanowires for light emission or piezoelectricnanowires for actuated surfaces or vibration or sound detection.Furthermore, nanowire structures with multiple shells can also befabricated, such as, for example, an undoped conducting inner channel, ahigher-energy doped inner shell, and an outer-shell dielectric. In thisway, carriers are contributed from the inner shell into the conductingchannel without providing dopants to the conducting channel that wouldreduce mobility through scattering. Such a structure can provideballistic transport devices, providing extreme-performancemacroelectronics. FIG. 8 illustrates a transmission electron microscope(TEM) and energy dispersive X-ray analysis (EDX) images of the crosssection and composition of a multi-shell nanowire with a germanium core,a silicon inner shell and a silica outer shell (see, e.g., L. J. Lauhon,M. S. Gudiksen, D. Wang, and C. M. Lieber Nature 420, 57 (2002)).

The ability to incorporate a conducting gate-electrode shell,conformally grown on the outside of each individual nanowire, candramatically increase TFT performance of DION films, reduce powerconsumption, and simplify the TFT fabrication process. Furthermore,integrating the gate electrode around the outside of each individualnanowire can provide a gate-electrode configuration for a cylindricalconducting channel (i.e., a conformal electrode that cylindricallysurrounds each individual semiconductor channel). This means that DIONthin-films can be fabricated with an intrinsic gate-dielectric andgate-electrode. Thus, the only post processing that is needed tofabricate electrical devices is to lithographically remove thegate-electrode shell from regions outside of the transistor channels anddeposit metal to connect the gate electrodes to the rest of the circuit.This slight increase in the complexity of the individual nanomaterialswithin a DION thin film (performed during nanowire synthesis) candramatically simplify the processing for fabrication of DION TFTs. Itcan also increase the performance of each DION TFT and decrease thepower consumption of overall DION electronic devices due to the idealgate contact configuration.

The materials described herein also have the inherent mechanicalflexibility of the high-mobility semiconductor material, which allowsfabrication of truly flexible high-performance electronics. Due to theextremely small diameter and large aspect ratio (>1,000), nanowirespossess superior mechanical flexibility and strength. Individualnanowires can easily bend with radius of curvature r<10 μm beforefailure. For example, FIG. 9 shows a dark-field optical micrograph (OM)of a nanowire with bend r<10 μm. The bar is 10 μm. The width of thenanowire artificially appears larger than it is due to the diffractionlimit of visible light. Because each individual nanowire on thehigh-density substrates described above is aligned in the samedirection, but physically independent of the surrounding wires,flexibility of the DION thin-film is retained. Even without bending theindividual nanowires within a device, the fact that each wire is only100 μm long allows a macroscopic r<<1 mm.

The materials described above are capable of being processed in asolution and have large-area compatibility. Unlike bulk semiconductorwafers, nanowires can be suspended in solution and then deposited andsecured onto virtually any substrate. This process is not limited to aparticular size range and is therefore ideal for large-area electronics.Combined with a flexible substrate, this technology will enablecompatibility with roll-to-roll production of high-performanceelectronics via nozzle or screen-printing technologies. One addedadvantage of this is the environment in which nanowires would bedeposited. Typical micrometer- and submicrometer-regime semiconductortechnologies require large clean rooms and specialized equipment withinthe clean rooms. Nanowires of the present invention can be suspended ina solution and then deposited onto large surfaces without the worry that“large” contaminants would disrupt the semiconductor wires. Defectcontrol can occur during the phase of fabricating the semiconductornanowires and preparing the solution, thus reducing the strictness ofthe printing process.

Other advantages include the ability to avoid the high-temperatureprocessing required for semiconductor deposition, annealing, orgate-dielectric deposition. DION TFTs can be fabricated on virtually anysubstrate (e.g., lightweight plastic). The high-T_(p) (temperature ofpeak crystallization) synthetic process used to make semiconductornanowires, gate-dielectric shells, and gate-electrode shells is doneoff-line (i.e., not in the presence of the substrate material).Therefore, extremely high-quality nanowires can be produced and thendeposited onto virtually any substrate material (even if the substrateis not compatible with high-T_(p) processing). In addition, since thesurface of nanowires can be treated chemically with any functionality, awide variety of substrates can be used.

Overall, by incorporating the extraordinary electronic and conductiveproperties of inorganic nanowires into dense oriented arrays on a solidsubstrate, a large-area, flexible semiconductor substrate can befabricated. Such a large-area, flexible semiconductor substrate canenjoy electrical performance comparable to or exceeding that of asingle-crystal silicon wafer in terms of mobility, threshold voltage,and I_(on)/I_(off). By incorporating other functional nanowirematerials, other functional devices with single-crystal performance canalso be fabricated.

Furthermore, such materials as mixed-composition DION thin-films (aUniversal Electronic Substrate Technology) can also be used. Multipledifferent nanowire materials can be deposited onto a single substrate atthe same time. Such mixed-composition DION thin-films can enable thefabrication of a single monolithic plastic substrate upon which it isfabricated an entire integrated electronic system (“System on a Sheet”).Unlike current devices fabricated on silicon wafers, such an embodimentof such a System on a Sheet could realize a variety of capabilities.These include, but are not limited to, high-performance CMOSelectronics, emit light like GaN, process high-frequency RF signals likeGaAs, vibrate to create or detect sound like a piezoelectric material,and include simple high-speed non-volatile memory throughspin-polarization like a ferroelectric thin-film. With mixed-compositionDION thin-films, all of these characteristics can be embodied into asingle monolithic substrate that can then be patterned and processedusing traditional lithographic technologies to fabricate an entirefunctional system on a rollable sheet of plastic. This technologyrepresents an unprecedented advance in the development of electronicsystems for military applications that are space, weight, and powerconstrained. (See, Table 1 for comparisons to other macroelectronicsmaterials). The technology described herein can leverage all of thedevelopments of uniquely functional single-nanowire electronic devicesthat have been developed so as to impart any combination of these uniquesingle-nanowire characteristics (or all of them) onto a singlemonolithic macroelectronic system on plastic.

TABLE 1 Comparison of traditional materials Mixed- Organic compositionSemi- Single- DION Functional Property conductors a-Si p-Si crystal Sithin-films Electron Mobility 0.001 1 100 1500 5000 (cm2/Vs) HoleMobility (cm2/ 0.1 0.1 30 500 1500 Vs) Threshold Voltage 2–10V 3V <2V<1V <1V (typical) Threshold Voltage extremely good poor ExcellentExcellent Uniformity poor On/Off ratio 10*5 10*9 10*6 >10*10 >10*6 CMOScompatible? No No Yes Yes Yes High-frequency RF No No No Yes Yescompatible? Light emitting No No No No Yes properties? Piezoelectric NoNo No No No properties? Ferroelectric No No No No Yes properties?

In the sections below, thin films of nanowires, and methods of makingthin films of nanowires, are described. This is followed by adescription of a DION TFT proof-of-concept demonstration and anexemplary CMOS TFT device fabrication process. Finally, variousapplications of thin films of nanowires are described. The applicationsdescribed below include the use of thin films of nanowires indistributed sensor networks, RFID tags, adjustable phase delays, andacoustic cancellation devices. Further applications for thin films ofnanowires are also described herein.

It should be appreciated that the particular implementations shown anddescribed herein are examples of the present invention and are notintended to otherwise limit the scope of the present invention in anyway. Indeed, for the sake of brevity, conventional electronics,manufacturing, semiconductor devices, and nanotube and nanowiretechnologies and other functional aspects of the systems (and componentsof the individual operating components of the systems) may not bedescribed in detail herein, but will be apparent to persons skilled inthe relevant art(s) in view of the description herein. Furthermore, forpurposes of brevity, the present invention is frequently describedherein as pertaining to a semiconductor transistor device. It should beappreciated that the manufacturing techniques described herein could beused to produce any semiconductor device type, and other electroniccomponent types. Further, the techniques would be suitable forapplication in electrical systems, optical systems, consumerelectronics, industrial electronics, wireless systems, spaceapplications, or any other application.

Nanowire Film Embodiments

The present invention is directed to the use of nanowires and nanotubesin systems and devices to improve system and device performance. Forexample, the present invention is directed to the use of nanowires insemiconductor devices. According to the present invention, multiplenanowires are formed into a high mobility thin film. The thin film ofnanowires is used in electronic devices to enhance the performance andmanufacturability of the devices.

FIG. 10 shows a close-up view of a thin film of nanowires 1000,according to an example embodiment of the present invention. Thin filmof semiconductor nanowires 1000 can be used instead of amorphous siliconor organic thin films in conventional electronic devices to achieveimproved device behavior, while allowing for a straightforward andinexpensive manufacturing process. Through the use of thin films ofnanowires, the present invention is particularly adapted to making highperformance, low cost devices on large and flexible substrates.

Note that thin film of nanowires 1000 as described herein may be formedin a wide range of possible surface areas. For example, thin films ofnanowires 1000 of the present invention can be formed to have functionalareas greater than 1 mm², greater than 1 cm², greater than 10 cm²,greater than 1 m², and even greater or smaller areas.

As shown in FIG. 10, thin film of nanowires 1000 includes a plurality ofindividual nanowires closely located together. Thin film of nanowires1000 can have a variety of thickness amounts that are equal to orgreater than the thickness of a single nanowire. In the example of FIG.10, the nanowires of thin film of nanowires 1000 are aligned such thattheir long axes are substantially parallel to each other. Note that inalternative embodiments, the nanowires of thin film of nanowires 1000are not aligned, and instead can be oriented in different directionswith respect to each other, either randomly or otherwise. In analternative embodiment, the nanowires of thin film of nanowires 1000 maybe isotropically oriented, so that high mobility is provided in alldirections. Note that the nanowires of thin film of nanowires 1000 maybe aligned in any manner relative to the direction of electron flow inorder to enhance performance as required by a particular application.

FIG. 11 shows a semiconductor device 1100 that includes thin film ofnanowires 1000, according to an example embodiment of the presentinvention. In FIG. 11, semiconductor device 1100 is shown as atransistor, having a source electrode 1102, a gate electrode 1104, adrain electrode 1106, formed on a substrate 1108. Thin film of nanowires1000 is coupled between source electrode 1102 and drain electrode 1106over a portion of gate electrode 1104. Thin film of nanowires 1000substantially operates as a channel region for the transistor ofsemiconductor device 1100, and allows semiconductor 1100 to operate withenhanced characteristics, as further described herein. Numeroussubstrate types applicable to substrate 1108 are described elsewhereherein.

Note that semiconductor device 1100 is shown as a transistor in FIG. 11for illustrative purposes. It would be understood to persons skilled inthe relevant art(s) from the teachings herein that thin film ofnanowires 1000 can be included in semiconductor device types in additionto transistors, including diodes.

In embodiments, the nanowires of thin film of nanowires 1000 are singlecrystal semiconductor nanowires that span all the way between sourceelectrode 1102 and drain electrode 1106. Hence, electric carriers cantransport through the single crystal nanowires, resulting in highmobility, which has not been obtained with current amorphous andpolysilicon technologies.

In addition, and without being bound to any particular theory ofoperation, due to a one-dimensional nature of the electron-wavetraversing inside the nanowire channel, and a reduced scatteringprobability, nanowires can be fabricated to achieve even higher mobilitythan a bulk single crystal material. Nanowires can be designed to be a“ballistic” transport for electrical carries. “Ballistic” is used hereinto mean transport through a nanowire with no scattering and where thenanowire has quantized resistance.

Note that a variety of contact area types can be formed forsemiconductor devices incorporating nanowires. The contact areas can beohmic and non-ohmic. For example, a non-ohmic Schottky diode barriercontact can be used as an electrode. A Schottky diode barrier contact iscommonly used for a III–V semiconductor material when it is difficult tomake a high quality gate dielectrics. Source electrodes 1102, gateelectrodes 1104, and drain electrodes 1106 are formed of a conductivematerial, such as a metal, alloy, silicide, polysilicon, or the like,including combinations thereof, as would be apparent to a person havingordinary skill in the art.

As described above, the nanowires of thin film of nanowires 1000 can bealigned or oriented. For example, the nanowires of thin film ofnanowires 1000 shown in FIG. 11 can be aligned parallel to the length ofthe channel between source electrode 1102 and drain electrode 1106, orcan be aligned in alternative ways.

Thin film of nanowires 1000 can be formed with a sufficient number ofnanowires to provide desired characteristics for semiconductor device1100. For example, thin film of nanowires 1000 can be formed of asufficient number or density of nanowires to achieve a desiredoperational current density or current level desired for the particularapplication. For example, the current level may be in the nanoamp range,including two nanoamps, and greater and lesser current levels. Forinstance, in the transistor example of FIG. 11, thin film of nanowires1000 can be formed to have a current level in the channel of greaterthan about ten nanoamps. By using larger numbers of nanowires, and/orhigher mobility nanowires, higher current levels are possible, includingcurrent levels in the microamp range, the milliamp range, and evengreater amounts.

For example, to achieve the required operational current density, aminimum number of nanowires can be included in the thin film ofnanowires for a given area on the substrate. Hence, each formedsemiconductor device will have a sufficient number of nanowires to carrycurrent at an operational current level. For example, the requirednumber of nanowires per unit area can be one nanowire, two nanowires,and any other greater number of nanowires, including 5, 10, 100, 1000 ormore.

In an embodiment, a thin film of nanowires 1000 can be formed to haveasymmetric mobility. For example, this can be accomplished byasymmetrically aligning the nanowires of thin film of nanowires 1000,and/or by doping the nanowires in a particular manner. Such asymmetricmobility can be caused to be much greater in a first direction than in asecond direction. For example, asymmetric mobilities can be created inthe order of 10, 100, 1000, and 10000 times greater in the firstdirection than in the second direction, or to have any other asymmetricmobility ratio between, greater, or less than these values. For example,this can be done by substantially aligning nanowires in a singledirection to create high mobility in the direction parallel to thedirection of the nanowires, and creating lower mobility a directionperpendicular to the direction of the nanowires.

The nanowires of thin film of nanowires 1000 can be doped in variousways to improve performance. The nanowires can be doped prior toinclusion in semiconductor device 1100, or after inclusion. Thenanowires can be doped prior to being formed into a thin film, or afterbeing formed into a thin film. A thin film of nanowires can be dopedafter being formed on the substrate. Furthermore, a nanowire can bedoped differently along portions of its long axis, and can be dopeddifferently from other nanowires in thin film of nanowires 1000. Someexamples of doping schemes for individual nanowires, and for thin filmsof nanowires are provided as follows. However, it will be apparent topersons skilled in the relevant art(s) from the teachings herein thatnanowires, and thin films thereof, can be doped according to additionalways, and in any combination of the ways described herein.

FIG. 12A shows a nanowire 1200 that is a uniformly doped single crystalnanowire. Such single crystal nanowires can be doped into either p- orn-type semiconductors in a fairly controlled way. Doped nanowires suchas nanowire 1200 exhibit improved electronic properties. For instance,such nanowires can be doped to have carrier mobility levels comparableto alternative single crystal materials. In addition, and without beingbound to any particular theory of operation, due to a one-dimensionalnature of the electron-wave traversing inside the nanowire channel, anda reduced scattering probability, nanowires can be fabricated to achieveeven higher mobility than a bulk single crystal material. Carriermobility levels up to 1500 cm²/V·s have been shown for single p-type Sinanowires, and carrier mobility levels up to 4000 cm²/V·s have beenshown for n-type InP nanowires.

FIG. 12B shows a nanowire 1210 doped according to a core-shellstructure. As shown in FIG. 12B, nanowire 1210 has a doped surface layer1202, which can have varying thickness levels. Thickness levels can beonly a molecular monolayer on the surface of nanowire 1210. Such surfacedoping can separate impurities from a conducting channel of thenanowire, and suppress an impurity-related scattering event, and thuscan lead to greatly enhanced carrier mobility. For example, whennanowires are doped according to the core-shell structure, “ballistic”transport can be achieved inside the nanowires. Further detail on dopingof nanowires is provided below.

FIG. 12C shows a nanowire 1220 that is uniformly doped, and coated witha dielectric material layer 1204, according to another type ofcore-shell structure. Dielectric material layer 1204 can be chosen froma variety of dielectric materials, such as SiO₂ or Si₃N₄. The use ofdielectric material layer 1204 can simplify fabrication of semiconductordevice 1100, as described elsewhere herein. The dielectric layer can beformed by oxidizing the nanowire, coating the nanowire, or otherwiseforming the dielectric layer. For example, other non-oxided highdielectric constant materials can be used, including silicon nitride,Ta₂O₅, TiO₂, ZrO₂, HfO₂, Al₂O₃, and others. Nitridation of nanowires canbe accomplished with processes similar to those employed in oxidation ofnanowires. These materials can be applied to nanowires by chemical vapordeposition (CVD), solution phase over-coating, or simply by spin-coatingthe appropriate precursor onto the substrate. Other known techniques canalso be employed.

FIG. 12D shows a nanowire 1230 that is doped with a doped surface layer1202 according to the core-shell structure shown in FIG. 12B, and isalso coated with a dielectric material layer 1204, as shown in FIG. 12C.

FIGS. 13A and 13B show examples of semiconductor device 1100, accordingto example doping embodiments of the present invention. As shown in FIG.13A, the top surface of substrate 1108 is coated with a dopant layer1302. Dopant layer 1302 includes electron-donor or electron acceptordoping materials. Properties of semiconductor device 1100 can becontrolled by the introduction of dopant layer 1302. The electron-donoror electron acceptor materials introduce negative or positive chargecarriers into the nanowires to achieve n- or p-channel transistors,respectively. Very high mobility levels can be attained in thisconfiguration for semiconductor device 1100 because the dopants areseparated from the actual conducting channel.

As shown in FIG. 13B, dopant layer 1302 covers a region of substrate1108 substantially localized around thin film of nanowires 1000. Inembodiments, dopant layer 1302 applied to semiconductor device 1100 canbe patterned to have two or more areas doped according to different n-and p-type characteristics. For example, in the embodiment of FIG. 13B,dopant layer 1302 has a first portion 1304 doped with an n-typecharacteristic, and a second portion 1306 doped with a p-typecharacteristic. In such an embodiment, a p-n junction can be achievedaccording to a variety of electronic and optoelectronic devices,including LEDs.

As described above, dopant layer 1302 can be introduced on substrate1108 prior to or after actual fabrication of semiconductor device 1100.

Collections of nanowires manufactured with these materials are usefulbuilding blocks for high performance electronics. A collection ofnanowires orientated in substantially the same direction will have ahigh mobility value. Furthermore, nanowires can be flexibly processed insolution to allow for inexpensive manufacture. Collections of nanowirescan be easily assembled onto any type of substrate from solution toachieve a thin film of nanowires. For example a thin film of nanowiresused in a semiconductor device can be formed to include 2, 5, 10, 100,and any other number of nanowires between or greater than these amounts,for use in high performance electronics.

Note that nanowires can also be used to make high performance compositematerials when combined with polymers/materials such as organicsemiconductor materials, which can be flexibly spin-cast on any type ofsubstrate. Nanowire/polymer composites can provide properties superiorto a pure polymer materials. Further detail on nanowire/polymercomposites is provided below.

As described above, collections or thin films of nanowires can bealigned into being substantially parallel to each other, or can be leftnon-aligned or random. Non-aligned collections or thin films ofnanowires provide electronic properties comparable or superior topolysilicon materials, which typically have mobility values in the rangeof 1–10 cm²/V·s. Furthermore, non-aligned collections or thin films ofnanowires can provide properties comparable or superior to singlecrystal material, if a sufficiently high density of nanowires is used.

Aligned collections or thin films of nanowires provide for materialshaving performance comparable or superior to single crystal materials.Furthermore, collections or thin films of nanowires that include alignedballistic nanowires (e.g., core-shell nanowires as shown in FIG. 12B)can provide dramatically improved performance over single crystalmaterials.

Aligned and non-aligned, and composite and non-composite thin films ofnanowires can be produced in a variety of ways, according to the presentinvention. Example embodiments for the assembly and production of thesetypes of thin films of nanowires are provided as follows.

Randomly oriented thin films of nanowires can be obtained in a varietyof ways. For example, nanowires can be dispersed into a suitablesolution. The nanowires can then be deposited onto a desired substrateusing spin-casting, drop-and-dry, flood-and-dry, or dip-and-dryapproaches. These processes can be undertaken multiple times to ensure ahigh degree of coverage. Randomly oriented thin films ofnanowires/polymer composites can be produced in a similar way, providingthat the solution in which the nanowires are dispersed is a polymersolution.

Aligned thin films of nanowires can be obtained in a variety of ways.For example, aligned thin films of nanowires can be produced by usingthe following techniques: (a) Langmuir-Blodgett film alignment; (b)fluidic flow approaches, such as described in U.S. patent applicationSer. No. 10/239,000, filed Sep. 10, 2002, and incorporated herein in itsentirety by reference; and (c) application of mechanical shear force.For example, mechanical shear force can be applied by placing thenanowires between first and second surfaces, and then moving the firstand second surfaces in opposite directions to align the nanowires.Aligned thin films of nanowires/polymer composites can be obtained usingthese techniques, followed by a spin-casting of the desired polymer ontothe produced thin film of nanowires. For example, nanowires can bedeposited in a liquid polymer solution, alignment can then be performedaccording to one of these or other alignment processes, and the alignednanowires can then be cured (e.g., utraviolet cured, crosslinked, etc.).An aligned thin film of nanowires/polymer composite can also be obtainedby mechanically stretching a randomly oriented thin film ofnanowires/polymer composite.

Thin films of nanowires can be formed on virtually any substrate type,including silicon, glass, quartz, polymeric, and any other substratetype describe herein or otherwise known. The substrate can be large areaor small area, and can be rigid or flexible, such as a flexible plasticor thin film substrate type. Furthermore, the substrate can be opaque ortransparent, and can be made from a conductive, semiconductive, or anon-conductive material.

Nanowire film contacts, including sources, drains, and gates, forexample, can be patterned on a substrate using standardphotolithography, ink-jet printing, or micro-contact printing processes,for example, or by other processes.

A dielectric layer can be applied to a thin film of nanowires on asubstrate to electrically insulate gate contacts, for example. Thedeposition of a dielectric layer can be done using evaporation, solutioncast of polymer or oxide dielectrics, and by other processes. Such adeposition of a dielectric layer on a substrate may not be necessary iftheir own dielectric layers insulate the nanowires.

Note that nanowire films can be patterned on a substrate using variousprocesses, including lithography techniques. Deposition and patterningof thin film of nanowires can be done simultaneously using variousprocesses, such as ink-jet printing or micro-contact printing methods.

Note that the order in which contacts are patterned can be varied. Forexample, gates 1104, sources 1102, and drains 1106 shown in FIG. 11 canbe patterned simultaneously with each other, or at different times. Theycan all be patterned prior to deposition of the thin film of nanowires1000, or afterwards. Sources 1102 and drains 1106 can be patterned priorto deposition of the thin film of nanowires 1000, while gates 1104 arepatterned afterwards. Alternatively, gates 1104 can be patterned priorto deposition of the thin film of nanowires 1000, while sources 1102 anddrains 1106 are patterned afterwards. Either of sources 1102 or drains1106 can also be patterned prior to deposition of the thin film ofnanowires 1000, while the other is patterned afterwards.

Note that in some embodiments, more than one layer of a thin film ofnanowires can be applied to a substrate in a given area. The multiplelayers can allow for greater electrical conductivity, and can be used tomodify electrical characteristics of a respective semiconductor device.The multiple layers can be similar, or different from each other. Forexample, two or more layers of thin films of nanowires having nanowiresaligned in different directions, doped differently, and/or differentlyinsulated, can be used in a particular semiconductor device. A contactarea of a particular semiconductor device can be coupled to any one ormore of the layers of a multiple layer thin film of nanowires. Note thata thin film of nanowires can be formed as a monolayer of nanowires, asub-monolayer of nanowires, and greater than a monolayer of nanowires,as desired.

DION TFT Proof-of-Concept Demonstration

In order to demonstrate DION TFT technology, a small-scaleproof-of-concept of a multi-nanowire TFT device is described. A SiO₂substrate is first patterned with a metal gate electrode (Ti/Au), whichis then coated with a thick layer (60 nm) of Al₂O₃ to form a gatedielectric. A moderate density film of heavily p-doped silicon nanowiresare then shear deposited on the substrate, leaving an array of roughlyoriented nanowires extending across the gate electrode. The surface isthen patterned and developed using standard E-beam resist to expose thesource and drain electrode regions. Aluminum is then deposited using astandard sputtering system. Because the nanowires are heavily doped, nocontact doping is required. The remaining resist is then removed,removing all of the wires that are not pinned down by the source anddrain electrodes. The transistor was tested by applying a 1 Voltsource-drain potential and sweeping the gate voltage. While theresulting device is rather non-uniform, with wires poorly aligned and inmany cases broken, the resulting transistor shows remarkably goodperformance, with an on/off ratio around 10⁴, and hole-mobility close to100 cm²/V·s. By increasing the number of nanowires per device, the totalon-current can be increased to as high as 1 mA, demonstrating thecapabilities of multi-nanowire TFTs (the basis of DION technology) forhigh-performance macroelectronic devices. FIG. 14 shows an overview of amultifunctional mixed nanowire thin film. The figure shows that alldifferent functions obtained in current nanoelectronic research can beflexibly integrated together on a single macroelectronic substrate.

These results coupled with an improvement in nanowire quality anduniformity, an incorporation of an intrinsic gate-dielectric andgate-electrode, improvements in deposition uniformity and density, andoptimization of electrical interface technology demonstrate that DIONTFTs can realize true single-crystal semiconductor performance on alightweight flexible plastic substrate.

As a breakthrough technology, DION TFT technology can benefit fromfurther development of materials, DION thin-films fabrication processes,and electrical interfacing and device fabrication processes. The resultsof these efforts will enhance the performance of DION TFT's inapplications.

Synthesis of multiple high-performance nanowire materials (includingn-doped silicon, p-doped silicon and n-doped InAs, InP and/or otherIII–V semiconductors) and fabrication of unique multi-layer core-shellstructures containing a semiconducting core, a gate-dielectricinner-shell and a gate-electrode outer-shell are specific areas in whichfurther developments will redound to the benefit of current DION TFTtechnology. Fundamental nanowire materials are a foundation of DIONtechnology. A substantial knowledge base of the fabrication andfunctionalization of unique nanomaterials, including inorganicsemiconductor nanowires already exists.

Film fabrication involves deposition technologies appropriate for thefabrication of mixed-composition DION thin-films. These include bothflow-based and dry-transfer technologies. Further developments willemphasize control of film density, nanowire orientation, filmuniformity, adhesion of nanowires to the film-substrate, and overallcost. Among different deposition technologies, a specific depositiontechnology can be identified that is compatible with the defined processflow for the complete device fabrication, and with the cost, size, andcomplexity requirements for devices for a distributed sensor networkapplication (described further below). This development will leverageexisting knowledge of the fabrication of low-density nanowire devicesfor applications such as chemical and biological sensors, and theassembly of nanowires into functional configurations.

Electrical interfacing involves processes for patterning and electricalinterfacing to DION thin-films to form DION TFTs, DION Schotky diodesand other devices. Further development of these processes can leverageexisting infrastructure and know-how developed for TFTs based ontraditional semiconductor materials, including photo-patterning,etching, doping, ohmic contact, etc. Among different processes, aspecific process can be identified such that all functionalities withinthe DION thin-film can be accessed using a single linear processcompatible with the end application, as well as antenna and sensordeposition (described below). This development will leverage currentknowledge of nanowire-based chemical and biological sensors, whichincludes the fabrication of high-quality and reproducible electricalcontacts to single-nanowire devices.

As noted, the present invention is a revolutionary and broad-reachingtechnology platform for the fabrication of multifunctionalhigh-performance macroelectronic integrated circuits on flexible plasticsubstrates over large-areas at low cost. This will be achieved byfabricating dense, oriented thin films of two or more differenthigh-performance inorganic semiconductor nanowires on a singlesubstrate. Each of these different materials can then be electricallyinterfaced to fabricate a variety of electronic and electro-opticdevices with performance equivalent to or exceeding that of the singlecrystal material from which the nanowires are formed. Hundreds orthousands of nanowires can span between each pair of source and drainelectrodes to produce a single crystal path across each device withthousands of independent conducting channels to ensure high carriermobility and high current capacity. FIGS. 15A, 15B, and 15C showpreliminary results for p-doped silicon nanowire thin-film transistors(μ˜100 and on/off ˜10⁴). FIG. 15A shows a dark field optical image of ananowire-TFT. FIG. 15B shows a transfer characteristic ofsource-to-drain current vs. source-to-drain voltage. FIG. 15C shows atransfer characteristic of source-to-drain current vs. number ofnanowires within the channel.

The general approach for the fabrication of DION TFTs comprises thesteps of: (1) synthesizing a batch of high-quality, single-crystalinorganic semiconductor nanowires with intrinsic gate-dielectrics andgate-electrodes incorporated into multiple shells around each wire; (2)harvesting the nanowires and suspending them in solution so they can betransferred to a desired substrate; (3) depositing the nanowires onto adesired substrate such that they form a dense monolayer (i.e., only onewire thick) thin-film with all nanowires substantially aligned in thesame direction; and (4) using standard semiconductor fabricationprocesses to pattern, develop, etch, and metallize the source, drain,and gate electrodes to form functioning DION TFT devices with transistorchannels running parallel to the direction of the aligned wires. Byusing nanowires that are much longer than typical macroelectronic devicedimensions (100 μm NWs vs. 10 μm channel-lengths), devices can beproduced wherein a large percentage of wires span the entire gap betweenthe source and the drain electrodes. Those few nanowires that arepositioned so that they only cross part-way may not contribute to theconductive properties of the device since the relative resistance ofcoupling between nanowires is many orders of magnitude higher than thehigh-mobility path through a single nanowire. FIG. 16 shows a schematicof a DION TFT.

An additional attribute of this new concept of macroelectronics is thatthe active semiconductor material growth is separated from the finaldevice supporting substrate. This not only enables the production ofTFTs at low temperatures to be compatible with plastic substrates, butalso allows the incorporation of two or more different types ofsemiconductors onto a single substrate. In this way, different uniquefunctional elements can be intimately integrated together to achievehybrid integration in a way not possible with conventional thin filmtechnologies.

In certain aspects, the present invention employs novel nanowirematerials and unique multi-layered core-shell nanowire structures tofacilitate the production of multiple high-performance functionalitieson a single substrate that can all be interfaced using a single lineardevice-fabrication process. In particular, three and four layercore-shell structures can be fabricated in which the interior core isformed from a semiconductor chosen to provide the desired electronicfunctionality (e.g., CMOS compatibility, RF signal processingcapabilities, light emission, etc.). The first shell is an insulatingdielectric layer that acts as the gate-dielectric in the final device.The outer layer is a conductive layer that acts as a conformal gateelectrode around each individual wire in the final device. In the caseof nanowires with III–V core materials for extreme mobility, which donot form a high-quality oxide or nitride, an intermediate layer (such asCdS) known to act as a high-quality interface between the semiconductorand the dielectric can be added. FIG. 17 shows schematics of amulti-core-shell nanowire comprising a semiconductor core, a passivationshell, an insulating gate dielectric shell, and a conducting gate shell.

The present invention includes at least three separate functionalitiesin DION thin-films by synthesizing three different nanowire materials:(1) high performance p-doped silicon (or SiGe) for PMOS circuits, (2)high-performance n-doped silicon (or SiGe) for NMOS circuits, and (3)extreme mobility n-doped III–V materials (e.g., InAs or InP) for RFsignal processing. Low power CMOS devices can also be realized bycombining the nanowire materials for the PMOS and NMOS circuits.Additional materials, such as electroluminescent nanowires,piezoelectric nanowires, or photovoltaic nanowires can also beintegrated into these films.

The fabrication of CMOS circuits and high-speed, high-frequency DIONelectronics for RF signal processing is of particular interest asneither of these functionalities is available today at any cost onflexible plastic using traditional technologies. CMOS circuits offersuperior performance and power-consumption for digital logic as comparedwith either NMOS or PMOS alone). Furthermore, CMOS technology ispervasive in modern electronic systems. Incorporation ofextreme-mobility III–V semiconductor nanowires into mixed-compositionDION films can facilitate low-cost RF signal processing, on the samemonolithic substrate, for power collection and active transmission andreception. Both of these functionalities are critical for a variety ofapplications, and specifically for distributed RF sensor networks.

There are several key technical abilities for fabricating the materialsfor using DION TFTs in distributed RF sensor networks. In particular,DION thin-film technology leverages previous work in nanowireelectronics that demonstrated the feasibility of making extremelyhigh-quality nanowire materials. However, these feasibility studies havefocused on single rather than multiple nanowire devices. In the case ofsingle-wire devices, wire-to-wire uniformity is not a concern, as eachdevice only samples a single nanowire. Wire-to-wire uniformity isimportant for DION thin-film technology. Additionally, distributed RFsensor networks require a substantial amount of nanowire material tocoat surfaces to realize electronics fabrication over an area of 4 ft².Thus, nanowire material needs to be fabricated on a relatively largescale.

An electronic device is only as useful as the material from which it ismade. The devices fabricated from DION thin-films require highhomogeneity within and between the nanowires that make up the underlyingthin-film semiconductor. In this case, this uniformity arises when eachdevice samples a homogeneous ensemble average of the nanowires within abatch (i.e., on average, each device contains the same number andcomposition of nanowires). Achieving this true ensemble sampling withineach device can be achieved by increasing the number of wires per device(in terms of the density of the nanowires or the size of the device) orby decreasing the variance in wire-to-wire uniformity (in terms ofoverall properties). Since practical limits exist for the maximum sizeof even a “macroelectronic” device (e.g., due to capacitive chargingrates) the uniformity of the nanowires and of the deposition processshould be optimized.

Synthetic process of nanowires should satisfy four conditions: (1) itshould be flexible enough to permit the synthesis of nanowires from anumber of different semiconductor compositions and dopant compositionsand concentrations, (2) it should be computer controlled andstandardized to eliminate user-to-user and batch-to-batch variabilitybetween samples, (3) it should be able to produce nanowires with uniformelectronic properties in the quantities needed for nanowire-based,large-area devices (i.e., capable of producing at least milligrams ofmaterial per batch), and (4) it should be inherently scalable.

The process should allow the precise control of all major physicalparameters of the nanowires (e.g., length, diameter, composition anddopant composition/concentration) both from wire to wire and from batchto batch. This can be realized by outfitting a manual synthetic processinto a standard computer controlled, commercial CVD oven. The processescan facilitate the incorporation of different semiconductor materialsand dopants (e.g., n-doped silicon or n-doped InAs). Using this system,materials synthesis can be optimized to produce nanowires with uniformelectronic properties that yield macroelectronic devices with acoefficient of variation of less than 10% for all relevant electronicparameters. In addition to improved uniformity of materials, acommercial CVD furnace can facilitate scaling the production ofnanowires for growth on 24 individual 4 in. wafers. This represents a2500-fold increase in volume over the manual synthetic process andallows further scalability as this technology is commercialized.

With controlled nanowire synthesis and device fabrication/testingmethods, the optimum materials for a specific macroelectronicstechnology can be determined by examination of the phase-space of avariety of materials. This can be done for each composition within themixed-composition DION thin-films. Nanowire-based macroelectronicdevices can have operating parameters that are quite different fromsingle-wire-based devices. The range of materials characteristicsnecessary for optimum performance can be predicted through theoreticalmodeling, but should be confirmed by synthesizing a sufficient number ofuniform nanowires to scan the parameter space of possible nanowirecompositions to find the best conditions for nanowire growth.

The synthetic process can also be used to develop methods for formingintrinsic, high-quality, gate-dielectric shells on the outside of eachnanowire and growing highly-doped amorphous silicon shells asgate-electrodes around the nanowire. A dielectric shell eliminates theneed for an external dielectric material and can greatly simplify thedevice-fabrication process. In addition, since the dielectric shell isformed off the substrate in a CVD reactor, this step can be done at ahigh-temperature, allowing a much higher-quality dielectric to be formedwithout subjecting the substrate to high temperatures. A conformal gatecan provide better performance than a flat-gate over these cylindricalchannels. In particular, the ability to apply an electric fielduniformly around each nanowire from all sides can result in much lowerthreshold-voltages and a steeper sub-threshold swing (since there willbe no variation in gate distance from each nanowire in a device).

With adequate process control and precision, as can be achieved with acomputer controlled CVD system, high-quality gate-dielectrics andintrinsic gate-electrodes on nanowires can be prepared with thenecessary levels of wire-to-wire and batch-to-batch uniformity. Thisprocess can be performed and optimized for all three types of nanowirematerials (described above), each requiring slight modifications to theshell configuration. In particular, an intermediate layer of CdS betweenthe III–V core and the outer shell dielectric layer can be included.

In addition to multi-layer core-shell silicon nanowires to optimizeperformance in mixed-composition DION films for CMOS circuits, groupIII–V nanowire materials without shells may be employed for certainapplications. Group IV semiconductors for CMOS, includinghigher-mobility SiGe nanowire cores, and III–V materials that include amulti-layer shell system and enhanced mobility can also be used. Alsoincluded in the present invention are processes for depositing DION thinfilms and mixed-composition DION thin-films onto an arbitrary substrateand for electrically interfacing to each region of the film. All ofthese steps should be performed with a specific emphasis oncompatibility with roll-to-roll processing. The general process stepsinvolved in fabricating a mixed-composition DION thin-film circuit aredepicted in FIG. 18.

A variety of different potential deposition processes can be employed,depending upon the desired density, alignment, speed, maximum depositiontemperature and maximum cost. Typically, such processes can beintegrated into a single linear fabrication process for patterning andprocessing the mixed-composition DION films to allow the completefabrication of mixed DION film devices using a single linear processthat is compatible with roll-to-roll processing and fabrication. Thisprocess can be dramatically simplified (and the cost reduced) by the useof the multi-shell nanowire structures described herein, which alreadyinclude the gate-dielectric and gate-electrode around each nanowire,eliminating the need for post-deposition of these layers. This processcan involve patterning the DION film with photoresist, to expose thesource and drain regions of the nanowires for each TFT, and etching toremove both the electrode material and the insulating layer. This leavesonly the semiconductor core exposed along with a self-aligned gateinherently integrated into the transistor channel. FIG. 19 schematicallyshows etching of shell materials in a multi-core-shell nanowire toexpose the core material for source drain contact in a nanowire TFT. Thegate is an inherently integrated shell material.

Within mixed-composition thin-films, there are multiple feasiblearchitectures that can be used to access the different materials using asingle linear monolithic process. These include, among others: (1)fabricating alternating stripes of different materials on a singlesubstrate, (2) fabricating a checkerboard pattern of different materialson a single substrate, (3) fabricating parallel and overlappingthin-films of different materials insulated from each other and etchingthrough the film during processing to access the layers of interest foreach device, and (4) fabricating a specifically designed substrate withmaterials deposited regionally to accommodate a specific electroniccircuit design. FIG. 20 shows several possible configurations of amixed-composition DION film. A number of approaches for deposition canbe used to produce these mixed-composition DION films, includingmultiple channel flow, photolithographic patterning, selectivechemical/biological patterning, contact-printing, color-ink-jetprinting, or screen printing.

For the overall patterning and processing of the deposited DION films,three types of exemplary roll-to-roll compatible processing can be used:(1) direct-write lithography, such as screen-printing, (2) traditionalTFT large-area macroelectronics photolithography, and (3) laserdirect-write lithography. All of these processes are adequate to achievesufficient circuit complexity at a low cost. Each has its own advantagesand disadvantages.

Photolithography is the process-of-choice for traditionalmacroelectronics. It has been used for many years in the fabrication ofequipment such as TFT-backplanes from a-Si for flat-panel displays.While photolithography is the most expensive option of the threeidentified above, it is still routinely used to fabrication TFT arraysat less than $30 per ft². The primary advantage to photolithography isthat it is possible to achieve relatively high resolution features (<1μm), which ensures sufficient circuit performance and complexity. Thesecond advantage is that photolithography using traditionalmacroelectronics steppers is an extremely well developed process, whichallows leveraging of the substantial expertise within the TFT industryfor process development. The main disadvantage of photolithography isthat it is a step-and-repeat process, such that any roll-to-roll processdeveloped will never be truly continuous. In addition, capital equipmentand operating costs for large-scale photolithography are much higherthan the other two processes, which substantially increases overallstartup costs.

Screen-printing for lithography has the extreme benefit of being fastand cheap. In addition, it is possible to develop a truly continuousroll-to-roll screen-printing process for depositing resist in acontinuous motion. In fact, today, certain manufacturers use high-speedscreen-printing to print their metal antennas across their passive RFIDtags at a rate of more than 1200 ft. per hr. for a 30 in. wide web. Theprimary disadvantage of screen-printing is resolution. While it has beendemonstrated that screen-printing can be used to generate feature sizesdown to 10 microns in size, traditional high-resolution screen-printinghas a resolution between 25 and 50 microns. The cost of capitalequipment and operation of screen-printing is low. One advantage of alow-resolution system such as screen-printing is that it is not asaffected by surface roughness or flatness of the flexible thin-filmsubstrate material. A lower resolution lithography process can not onlydecrease the cost of production and increase the speed of production,but also simplify development of components to handle and positionflexible substrates for high-resolution lithography. This can facilitatehigh yield fabrication processes.

Similar to screen-printing, a scanned laser rastered back and forthacross a web traveling perpendicular to the direction of the scan isable to expose a traditional photolithographic resist in a continuousmanner at a low cost and a relatively high speed. In addition, such asystem allows for higher-resolution than traditional screen-printing.Laser-scanning is not as fast as screen-printing, or as high-resolutionas step-and-repeat photolithography, but it is a reasonable intermediatebetween the two. Such systems are currently available commercially for anumber of applications. They can also be used to perform contactannealing or laser activation of dopants for source-drain contacts.

The particular process employed will often depend upon the particularsof the material and application to which the material is being put.Typically, factors such as resolution, cost, speed and roll-to-rollcompatibility are taken into account in selecting a process.

Vapor-deposited metal electrodes can be used to fabricate DION TFTs.Fortunately, commercial roll-to-roll metal vapor-deposition systems arereadily available. The specific metal and post-processing can bedetermined empirically based on performance results, theoreticalmodeling, and an existing and extensive knowledge base in fabricatingelectrical contacts to single-nanowire devices. The most difficultelectrical contact is the gate electrode that conformally coats aroundeach wire to provide extreme uniformity in threshold voltage andextremely low absolute threshold voltage. Fortunately, the synthesis ofthe underlying multi-layer nanowire materials that are the buildingblocks of the DION films accounts for fabrication of the gate electrode.During processing, only a thin-layer of patterned metal is applied tothe region of the gate electrode contact to make ohmic contact to theouter conductive gate electrode shell. Source and drain contacts are adifferent issue. In the case of mixed-composition DION thin-filmscontaining heavily doped n- and p-doped nanowires, high-quality ohmiccontacts between the metal and semiconductor can be formed directly.This is a unique capability for DION thin-films, which cannot beachieved with other materials.

Inversion-mode devices, which use lightly-doped nanowires and the gatevoltage to define the majority carrier, can also be fabricated. In thiscase, the source and drain contact regions of each device should bepost-doped before metallization. This can be done through eithertraditional ion-implant processes (a standard in the TFT industry) orthrough solution deposition of either n- or p-dopants onto exposedregions of the DION film followed by laser-activation of each region.The latter process is appealing in that it does not require any elevatedtemperatures and it has been successfully used in the fabrication ofsingle-wire devices. Ion-implant processes can also be used,particularly with low-temperature process parameters.

The universal electronic substrate concept is used to produce highperformance CMOS circuits by hybrid integration of different highquality p- or n-type semiconductor materials onto a single substrate atthe device level. FIG. 21 schematically illustrates an example of usinga mixed-composition DION thin-film to form a CMOS circuit. This hybridintegration allows arbitrarily choosing among high mobility p-type andn-type materials. These materials can be integrated together to realizea substantial improvement in performance. Hybrid integration ofdifferent semiconductor materials have been intensively researched inthe bulk semiconductor materials, but only with limited success due toserious issues in lattice mismatching and process compatibility. On theother hand, nanowire thin film technology readily facilitates hybridintegration since different high quality single crystal semiconductornanowire materials can be synthesized separately and flexibly assembledonto a single substrate over a large area.

The following technical abilities should be established in order to useDION technology to manufacture practical devices in commercialquantities: (1) synthesis of engineered multi-shell nanowires at asufficient scale and uniformity, (2) the deposition of nanowires to formmixed-composition, dense, robust, highly-oriented thin films with auniformity and reproducibility necessary to enable the fabrication oflow-cost, high-performance, high-yield electronics, and (3) optimizationof the electrical interface for both ohmic and insulated electricalcontact to each device within a mixed-composition DION film (i.e., thenano-to-macro world interface). In addition, there are other technicalabilities related specifically to a distributed sensor networkapplication.

Synthesis of p- and n-channel nanowire materials with specified materialparameters is included in the present invention. Of particular interestare materials such as Si, Ge, and an alloy of Si and Ge for p-channelmaterials since these are materials of high hole mobility. Inparticular, the hole mobility of ˜1900 cm²/V·s for Ge is highest of allsemiconductors in bulk form. For n-channel materials, Si and Gematerials are also important. In addition, other high electron mobilitymaterials including group III–V materials (e.g., InP: μ_(e)˜5000cm²/V·s, InAs: μ_(e)˜30,000 cm 2/V·s) can be used for highelectron-mobility TFTs. Multi-core-shell nanowire structures can be usedfor modulation doping to separate dopants from the active conductingchannel for ultra-high carrier mobility. The nanowire surface can alsobe passivated using various core-shell structures and can be furnishedwith an outer gate dielectrics shell and a circumferential conductinggate shell.

Silicon nanowire synthesis can be carried out using a gold nanoparticlecatalyzed CVD process. Briefly, a predetermined precursor gas mixture,SiH₄ and B₂H₆ or PH₃ in He, can pass over catalyst-gold particlesdeposited on an oxide coated silicon substrate at a total pressurebetween 20 and 50 Torr while the gold nanoparticles are heated up to atemperature of about ˜450° C. Upon contact with the gold nanoparticles,SiH₄/B₂H₆/PH₃ will decompose and silicon and boron (or phosphorus) atomscan diffuse into the gold nanoparticle and generate a liquid alloydroplet. As the droplet begins to saturate with these precursors, Si/B(or Si/P) atoms will precipitate out and initiate nanowire growth. Bycontinuously supplying SiH₄ and B₂H₆ (or PH₃), nanowire growth canproceed indefinitely until the process is terminated. The quality of thenanowires is dependent on the quality of the gold nanoparticles, thediameter distribution of the gold nanoparticles, and the growthconditions, including temperature, ratio of SiH₄ to B₂H₆ or PH₃, partialpressure of the SiH₄, and resident time of the precursor gases in thereactor. The growth can be carried out using a computer controlled 8 in.semiconductor furnace using 4 in. silicon oxide coated silicon wafers asthe substrate.

Most semiconductor materials have a significant surface state densitydue to dangling bonds and some trapped charges at the surface. Due totheir extreme surface-to-volume ratios, nanowires are much more affectedby these surface effects, which can significantly limit deviceperformance. Methods to deal with the effects of surface states havebeen well developed in the semiconductor industry for traditionalsemiconductors. A number of strategies are used to minimize the affectof surface states, including direct thermal annealing under an inert orhydrogen/forming gas atmosphere, and annealing in hydrogen plasmafollowed by thermal anneal or rapid thermal annealing. Each of theseprocesses can be done off-line, before the nanowires come into contactwith the plastic substrate material.

As an alternative and general approach, a core-shell structure can beused as a generic approach to passivate any surface trapping states. Inthis case, the shell can be selected to be a larger bandgap materialthan the core so that carriers cannot penetrate the shell and aretherefore electrically insulated from any surface environment. Inaddition, a high quality insulating passivation shell can also beemployed as an integrated gate dielectric layer in TFT devices. Thequality of the dielectric coating can be a key factor in determining theperformance of nanowire-TFTs. In particular, the threshold voltage andleakage current can be primarily determined by the quality and thicknessof the shell and the semiconductor-shell interface. It is important tobalance these two parameters, by producing a defect-free shell as thinas possible.

In the case of silicon, the issue of surface passivation and gatedielectrics can be simultaneously addressed by using a high quality SiO₂coating. Methods and underpinning scientific principles for high qualitydielectric coating have been well established for planar single crystalsilicon and, lately, for amorphous and polysilicon. Pinhole-free gatedielectrics with uniform thickness around the surface of the nanowires(which samples several distinct crystallographic directions) can also beformed. In general, methods can be categorized into direct oxidation orCVD deposition for silicon oxide dielectrics. Direct oxidation isparticularly compatible with the unique structural nature of siliconnanowires.

The direct oxidation of 60 nanometer diameter silicon nanowires can becarried out in the nanowire growth furnace. After nanowire growth isterminated, the reactant gas mixture is pumped from the reaction tube,which is replenished with a mixture of oxygen (5%) and helium to apressure between 100 mTorr and 760 Torr at a temperature below 150° C.The temperature of the furnace can then be slowly raised to between 300°C. and 800° C. The oxidation temperature, together with the ratio ofoxygen to helium, the partial pressure of oxygen, and the duration ofoxidation determines the thickness and the quality of the generatedsilicon oxide. These conditions can be optimized until a desiredthickness (2–20 nm) and coating quality is obtained. (Recall that FIG. 7illustrates a semiconductor/dielectric core-shell structure.) A slowoxidation is desired in order to minimize defects and dangling bondswhich can result in trapped charges.

If pinhole defects or the inability to stop dopants from the gate frommigrating into the oxide render a simple oxide layer insufficient to actas a good gate-dielectric, then direct nitridation of silicon dioxidecoated nanowires can be used to generate a more stable oxynitridecoating. The higher dielectric constant and lower permeability to mobileions can make oxynitride a better gate material. Alternatively, a purenitride layer can be formed with an even higher quality and dielectricconstant. In the case of nitride or oxynitride, a plasma assisted directnitridation method can be employed using NO or NH₃.

To further enhance device performance and simplify the devicefabrication process, a conducting-shell surrounding the gate dielectricscan be used, which can function as a circumferential gate. For example,in the case of a Si/SiO₂ core-shell structure, a doped amorphous orpolysilicon shell can be deposited onto the Si/SiO₂ core-shell nanowiresto realize a circumferential gate. This can be done as an added step inthe CVD reactor, following oxide or oxynitride formation. In order toensure that the silicon electrode deposits onto the surface of the wire,rather than resulting in continued nanowire growth, the catalyst colloidshould be removed prior to the gate-electrode deposition. This can bedone with a metal etch of the wafer prior to silicon deposition.

Single crystal silicon nanowires offer the opportunity to produce TFTswith performance approaching that of single crystal materials. However,the ultimate performance of these materials is limited by the intrinsicproperties of silicon materials. A major advantage of the nanowires ofthe present invention is the ability arbitrarily to incorporate anymaterials into final device applications. New nanowire materials can beidentified and synthesized as necessary to further enhance the mobilityof the materials, particularly for high-speed, high-frequencyapplications. Specifically, n-type materials should be synthesizedsince, in semiconductor materials, electron mobility is greater thanhole mobility. The selection of good candidates for high electronmobility can be based on a number of bulk material properties includingintrinsic electron mobility, work function, and surface properties.Surface properties are particularly important because of the largesurface to volume ratio of nanowire materials. III–V group materials areparticularly good candidates since they generally have high electronmobility (e.g., μ_(e)˜5000 cm²/V·s for InP, and ˜30,000 cm²/V·s forInAs), and are currently widely used for high-speed electronics.Additionally, these materials have useful optical properties and can beused for macroelectronic LEDs and infrared (IR) detectors using a DIONthin-film platform. InP nanowire materials are discussed further below.InP is an especially useful material due to its high electron mobilityand its moderate surface properties. Generally, the synthetic process ofthe present invention can be applicable to many different materials.

A metal (e.g., gold) cluster mediated approach can be used to synthesizeInP nanowires with a system as illustrated at FIG. 22. FIG. 22 shows aschematic of a generic reactor for fabrication of semiconductornanowires. Thermal heating or pulsed laser heating can be used togenerate a vapor of InP precursor from InP powder or a solid InP target.The gaseous precursor can be carried over to the substrate by a carriergas and can react with Au colloid nanoparticles to produce nanowires ina similar way to that of silicon nanowires. The diameter of the Aucolloid can be used to control the diameter of the resultant nanowires,and the growth time can be controlled to produce nanowires with adesired length. The doping can be controlled by the amount of the dopantsource in the raw materials. Synthesis of InP nanowires can be furtherdeveloped leveraging the existing knowledge base for the synthesis ofsilicon nanowires. Pressure, flow rate, and temperature can be carefullycontrolled to determine the material properties. The morphology andelectronic properties of InP nanowires can be characterized in a similarmanner to that of silicon nanowires. Co-deposition of In and Pprecursors, as well as dopants, can be used to achieve stoichiometricdeposition.

Physical and chemical characterization is important to gauge thematerials properties of synthesized nanowires, including morphologies,diameter, length, chemical compositions, and overall uniformity, and todirect further optimization of nanowire synthetic protocols. SEMs andatomic force microscopes (AFMs) can be used for characterization of thelength and diameter distribution of the nanowires. High resolution TEMscan be used to measure the thickness and uniformity of the dielectriccoating and the quality of the crystal lattice of individual wires.X-ray diffractometry can be used to measure overall crystal quality andorientation of the films. An EDX attached to a TEM can be used to assessthe chemical composition of single-nanowires. Specific software suiteshave been developed for rapid physical characterization of nanowires.

Single nanowire FETs have been demonstrated. However, largemanufacturing variations from device to device have delayed use of thesedevices in practical applications. These variations can be due to a lackof synthetic control, reliable electrical contacts, and/or large numbersof random surface trap states. Achieving reliable and controllableelectrical characteristic is important for any commercial or militaryapplication of DION thin-film technology. In order to achieve a highlevel of control over device characteristics, the electronic propertiesof individual nanowires should be highly reproducible and controllable.Electronic quality control of the nanowires can be characterized andoptimized using a single nanowire FET structure, since this allows for acomprehensive analysis of materials statistics (e.g., no ensembleaveraging). With well-controlled electronic properties, single-nanowireTFT devices can be fabricated and characterized on different substratesincluding flexible plastics. This process can use methods forcontrollably penetrating the gate-dielectric shell in areas where sourceand drain electrodes are to be applied, and for potentially doping theseregions to improve electrical contact. Methods for deposition of gateelectrode materials that do not penetrate the gate-dielectric shell canalso be used. However, this process can be risky due to the smallcontact area per nanowire and the high etch-resolution it requires.

Single nanowire FETs can be used to test metallization processes for usein the fabrication of DION TFTs. Single nanowire FET devices can befabricated on a SiO₂/Si surface using either electron-beam lithographyor photolithography. A standard lithographic platform has been developedfor electrical evaluation of single nanowire devices. FIGS. 23A and 23Billustrate a standard nanowire FET testing platform. FIG. 23A shows asingle dye on a 4 in. wafer containing 12 individual devices withdifferent gate widths and lengths. By controlling the density ofnanowires over these electrode pairs, single nanowire devices ofdifferent configurations can be tested. FIG. 23B shows alow-magnification of an entire wafer full of test-devices. The mask setsand processes used for this test pattern were originally developed forevaluation of single nanowire devices, but can also be used forevaluation of DION films for rapid feedback of materials performance.

Silicon substrate can be used as a global back gate, and two metalelectrodes can be used as source and drain electrodes. Planarsemiconductor technology can be used as a reference point for choosingan appropriate contact metal, etchant and device fabrication protocol.Prior to metallization, appropriate surface cleaning procedures can beperformed to remove the dielectric shell from the nanowire surface andto ensure a good contact between the nanowire and the contact metal.Various strategies, including ion gun cleaning and hydrofluoric acidetching, can be employed to remove the surface dielectrics prior tometallization of source-drain electrodes. Different metallizationrecipes (e.g., Ti/Au, Ni/Au, Al, In) can be tested and optimized usingeither electron-beam evaporation or sputtering process. The devicebehavior can be characterized using a semiconductor analyzer. Variousmeasurement configurations, including gate-dependent two-terminalmeasurements and four-terminal measurements, as well as electric forcemicroscopy can be employed to characterize the device behavior. Theresults from the electrical test can further be used as feedback tooptimize nanowire synthetic process and metallization processes until areliable procedure is obtained. This can be the standard metallizationprocess for the fabrication nanowire-TFTs. A single nanowire FET canalso be used for nanowire qualification tests. A database can beconstructed in terms of synthetic conditions and the electronicparameters of nanowires. The database can be further used to guidedevelopment of more controllable synthesis and device fabricationprocesses.

Because high-temperature annealing processes are not compatible withplastic substrates, direct metal contacts are preferable to p/n diodecontacts, which are normally used in conventional FET or TFT fabricationprocesses. Different metals can be tested for different semiconductormaterials based on existing knowledge in the semiconductor industry.Sufficient analysis of the work function the nanowire materials, as wellas various metal candidates, can be carried out to identify the bestcontact metals. Particular attention can be paid to identifying a commoncontact metal for different nanowire materials (e.g., Si and InPnanowires) since device fabrication can be greatly simplified if asingle metallization process can be used in a mixed nanowire thin film.

DION TFTs fabricated with different surface densities of nanowires canbe demonstrated and characterized using a similar device structure asthat of the single-naowire FETs. DION thin film can be deposited usingprotocols as described below. With a reliable protocol identified forthe fabrication of single nanowire devices, this protocol can be appliedto DION TFTs. TFT devices can be fabricated using DION thin films withdifferent surface densities to achieve individual TFT devices withvariable number of nanowires bridging the source and drain electrodes. Asemiconductor analyzer can be used to characterize behavior of thedevices, such as current level, on/off ratio, threshold voltage, andgate leakage current as a function of nanowire surface density. Thebehavior of the device can be theoretically modeled to calculatecritical device parameters, including carrier mobility values. Themodeling can in turn be used to direct the design of device structuresto achieve desired device functions. These studies can be carried out onSiO₂/Si or Si₃N₄Si substrate using silicon substrate as the back gate,as this is an easy way to realize device fabrication and modeling. Areliable protocol can be formed to fabricate DION TFTs with variablenanowire surface density and controllable device behavior. Statisticalanalysis of a large number of devices can be used to determine theminimum density of nanowires required on a substrate for a given devicefeature-size to avoid the effect of statistical fluctuations in thenumber of connections from impacting device reproducibility.

Once optimum deposition and materials characteristics are selected for agiven application, the same analysis can be performed using anindependent gate electrode deposited either underneath or over the topof the nanowires between the source and drain electrodes. FIG. 24schematically illustrates a DION thin-film transistor using a globalback-gate. For testing, the silicon substrate on which films are formedcan be used as the global gate. This configuration represents a fullyfunctioning DION TFT, and can mimic the format used in the next step asthe materials are transferred to plastic (for which no global back-gatecan be used).

Locally-gated TFT structures can be fabricated usingsemiconductor/dielectric core-shell nanowires, where the dielectricshell is used as the gate dielectrics and an additional metal electrodecan be employed as the gate electrode. FIG. 25 shows a schematic view oflocally gated nanowire thin film transistors. This configuration can betested to measure switching voltage, on/off ratio, leakage currents, andreproducibility. All of these tests can be done first on plasticsubstrate, such as polyetheretherketone (PEEK) or polyethyleneterephthalate (PET). In order to achieve reliable device performance,the surface roughness of the plastic substrate should be minimized by,for example, coating with a layer of cured SU8 photoresist.Additionally, surface modification with thin oxide coatings such as SiO2or Al2O3 can be used to improve device adhesion to the plastic surface.

Theoretically, the structure of the device can be sufficiently modeledto derive key transistor parameters, including carrier concentration andmobility, threshold voltage, on/off ratio, etc. In particular, resultsobserved from both single-nanowire devices and DION TFTs fabricated fromthe same materials can be compared to completely understand theinfluence on wire-to-wire variations and characteristics on the ensembledevice performance. The results from the electrical test and theoreticalmodeling can further be used as feedback to optimize nanowire synthetic,deposition, and metallization processes.

The materials and substrates described herein can be processed using acost-effective process for the fabrication of high performance CMOScircuits on a mixed nanowire thin film by using the high qualitymaterials described above.

Oriented nanowire arrays are important in ensuring a high-mobilityconducting channel between the source and the drain of the nanowire-TFT.To obtain highly oriented nanowire thin films over a large area, anumber of strategies can be used, including shear alignment, fluidicflow alignment, electrical field alignment, Langmuir-Blodgett film, andinkjet printing. Fluidic flow and shear nanowire deposition over largearea substrates can generally be employed into an approach to suchprocessing that is compatible with roll-to-roll processes. Prior tonanowire thin film deposition, generic surface modification approachesto modifying the substrate and nanowire surface are typically desirableto ensure a complementary interaction and a stable thin film formation.

Since most of the nanowire materials described herein can be terminatedwith a (native) silicon dioxide shell, alkyl-trimethoxysilane cangenerally be used to attach an alkyl chain to the nanowire. The terminalgroup of the alkyl chain can be controlled to yield either a hydrophobicor hydrophilic surface, or a surface with a special function group to becomplementary to other function groups on a substrate. In the case ofnanowires terminated with other surfaces, different chemicals can beuses when necessary. For example, for InP or CdS, the trimethoxysilanehead group can be replaced with a thiol group to provide robust surfacefunctionalization.

Surface modification can facilitate subsequent thin film deposition ontosubstrates like plastics. However, there is a risk that these surfacegroups may have detrimental effects on the electronic properties ofnanowires. The impact of different shell and core-shell structures canbe explored to minimize the impact of surface ligands on the electronicproperties of nanowires. In addition, methods for removing the organicmolecules from nanowire surfaces subsequent to nanowire thin filmformation can also be employed before patterning and electrodedeposition. Techniques such as oxygen plasma or ozone cleaning processesappear to be worth exploring.

The surface chemistry of the substrate material is also important and ausefully controlled parameter for proper adhesion of nanowires to thesubstrate surface. Since all nanowire surfaces can be terminated with anoxide shell, the surface chemistry required for the substrate is similarto that needed for the adhesion of the various nanowire materials.Hydrophobic plastic substrates can be modified with an oxygen plasmaoxidation process followed by attachment of a monolayer of3-aminopropyl-group to the surface using 3-aminopropyltrimethoxysiline.It is also possible to modify the surface by first coating the plasticsurface with a thin layer of SiO₂ flowed via standard SiO₂ surfacemodification chemistry. If adhesion is found to be a problem, disiloxanecompounds can be used to anchor the nanowires to the surface. Ifnecessary, these organic molecules can be removed after metallization,at which time the electrodes will pin nanowires to the substratesurface. In such an anchoring method, careful control of flocculation ofthe nanowires prior to deposition is required, since a chemical compoundthat can bind SiO₂ nanowires to a SiO₂ surface can also bind them toeach other. This can be resolved by treating the substrate surface withthe anchor and then removing excess prior to deposition, or by using agate-dielectric shell that has a chemical reactivity different from thatof SiO₂.

The fluidic flow approach has been applied to align nanowires atlow-densities for nanoelectronics applications over widths of hundredsof micrometers and lengths of a few centimeters. Fluidic flow alignmentcan be extended to very large areas. In order to achieve alignment overlarge areas, a fluidic channel with a lateral dimension comparable tothe substrate size should be used. The height of the channel can becontrolled to be less than 500 μm so that a major portion of thenanowire solution is proximate to the substrate surface. The shear flownear the surface of the substrate can enable the alignment of thenanowire along the flow direction. Different nanowire solutionconcentrations and times can be used to control the nanowire surfacedensity on the substrate. When desired, the substrate can also befunctionalized to enhance the complementary interaction between thesubstrate and the nanowires to achieve higher surface coverage. Asystematic study can be conducted to enable reproducible nanowiredeposition on a surface. The surface coverage can be studied with anoptical microscope and an SEM, and a rational statistical approach canbe developed to quantitatively characterize the surface coverage.Density can even be monitored optically in real-time during thedeposition process by imaging the scatter from the nanowires through adark-field microscope for feedback to the flow-system. These studies canbe first conducted on a glass substrate and then implemented on theplastic substrates.

Several critical issues should be noted and carefully controlled: (1)the rubber stamp poly-dimethylsiloxane (PDMS) fluidic channel used forsmall scale alignment may not be applicable at inch- totens-of-inch-dimension due to the flexible nature of PDMS. To overcomethis issue, a solid channel can be employed using glass or stainlesssteel. The perimeter of the channel can be sealed using either an O-ringor a thin layer of PDMS. (2) At such a large dimension scale, care willbe required to insure that the flow across and along the whole channelis uniform. Particular attention should be paid to design of the fluidicchannel entrance and outlet. Great care should also be taken to designthe solution delivery scheme. A programmable pump can be used to ensurea constant solution delivery rate. Without further precautions, it islikely to deposit a much higher nanowire density in the area near thechannel entrance than near the outlet, which is often observed inmicro-channel fluidic alignment without careful design of the channelentrance. This density variation can largely be compensatedalternatively by reversing the flow direction during the alignmentprocess and by enhancing the interaction between nanowires and thesurface of the substrate through chemical functionalization.

To achieve mixed nanowire thin films that compromise two or more typesof nanowires (e.g., p- and n-type silicon nanowiress, or Si and InPnanowires). Deposition is an important step. Approaches used for singlecomposition oriented nanowire can be leveraged and modified. In oneembodiment, photolithography can be used and be followed by amultiple-step fluidic flow assembly approach to achieve a mixed nanowirethin film. In another embodiment, more complicated approaches for asingle step mixed thin film deposition can be used. An example processto form a mixed nanowire thin film can include: (1) the substratesurface is patterned using photolithography with some region exposed toreceive a first type of nanowire thin film and another region coveredwith photoresist, (2) a first type of nanowires (e.g., p-Si) is appliedto the substrate using a fluidic flow approach, (3) lift-off isperformed to have a substrate with some region of the surface coveredwith a p-Si nanowire thin film, and (4) the nanowires are anchored bydepositing a gate metal. Steps (1) to (4) can be repeated to assemble adifferent type of nanowire (e.g., n-Si) onto a different region of thesubstrate surface to produce a mixed nanowire thin film.

Device fabrication over a large area is important to macroelectronicapplications. There are several possible barriers to large area devicefabrication, particularly on plastics. First, the device fabricationprocess should be carried out below the glass transition temperature(T_(g)) of the plastics. To this end, a low-temperature devicefabrication process with a maximum process temperature <100° C. can bedeveloped which is compatible with most plastic substrates. Second,there are expected issues in alignment in large area lithography due tothe flexibility of the plastic substrate. This can be addressed by usinga relatively rigid plastic substrate with a thickness above 1 mm.Thinner substrates can be laminated onto glass substrates during thefabrication process. When the device fabrication process is completed,the plastic substrate can be released from the glass substrate to obtainflexible electronics. Finally, to simplify the overall devicefabrication process, the knowledge base of existing fabricationtechnology can be leveraged. Preferably, a single metal can be used tomake contact to both types of nanowire thin films. Lower-costapproaches, including inkjet printing, screening printing, or scanninglaser lithography, can be assessed to determine which are compatiblewith roll-to-roll large-volume production process.

Photolithography has been routinely used for micro- and macroelectronicsfabrication. An example photolithography-based process for devicefabrication on mixed nanowire thin film includes: (1) deposition of amixed nanowire thin film, (2) formation of patterns usingphotolithographic processes and metallization of gate electrodes, (3)etching of a shell layer in core-shell nanowires and metallization ofsource and drain electrodes.

A 12-stage shift register having approximately 104 transistors can bebuilt. A target clock speed for the shift register is 25 MHz. In orderto provide this level of performance on a large-area, flexiblesubstrate, silicon nanowires can be used in a core-shell structure.Preferably, electron and hole mobilities of, respectively, 400 cm²/V·sand 200 cm²/V·s can be achieved with a lower limit of, respectively, 100cm²/V·s and 100 cm²/V·s. Available data on single-nanowire mobilitiesindicate that the mobilities achievable for these silicon wires could beas high as, respectively, 1500 cm²/V·s and 400 cm²/V·s. Based on theactual achieved mobilities with CMOS silicon DION TFTs, the resolutionof lithography required to achieve a 25 MHz clock speed can bedetermined and a specific lithographic process (i.e., photolithography,scanning laser lithography, and jet/screen printing) can be selected. Anexemplary process is described below.

In a first step, nanowires are grown in a nanowire reactor. The VeryLarge Scale process described above can be used to provide 80 nmdiameter silicon nanowires with doping of about 10¹⁸ per cm³. Subsequentthermal oxidation can reduce the diameter to approximately 60 nm andprovide the bulk of the gate insulator. A two-step nitridation processcan then be used to produce silicon nitride-rich diffusion barriers,which guard against threshold voltage shifts due to mobile ionpenetration. This can bring the gate oxide to its final thickness ofapproximately 20 nm. A final synthetic step can be to grow a dopedplasma enhanced chemical vapor deposition (PECVD) amorphous siliconlayer around the surface of the nanowires. This provides a conformalgate contact to the gate oxide with a low barrier potential differencewith the silicon channel, and hence a lower threshold voltage. Thechannel doping can be relatively high so that the threshold voltage willlikewise be relatively high. At this point, the nanowires comprise athree-layer core-shell, with single-crystal silicon in the center,surrounded by an oxynitride gate dielectric, which in turn is coatedwith a doped amorphous silicon conformal gate electrode.

The nanowires can then be removed from the reactor, harvested, anddeposited on a flexible PEEK plastic substrate using the depositionprocess described above for mixed-composition DION thin-filmfabrication. Using the lithographic process described above (e.g.,standard photolithography or screen-print-lithography), gate metalcontacts can then be patterned and used as a mask to remove the exposedamorphous silicon. Amorphous silicon will remain only under the gatecontact (i.e., this can be a self-aligned process, which can reducelithographic complexity and cost and power consumption and increasedevice performance). The source and drain contact areas can then bepatterned and the gate oxide can be etched off in the source and draincontact areas using a standard oxynitride etch. Ohmic contacts can beapplied for the source and drain through the resist pattern using E-beamevaporation. Based on previous experience with single-nanowireelectrical interfacing, only low-temperature (<150° C.) or no contactannealing should be required. Finally, the entire device can bepassivated with a layer of silicon nitride. If necessary, excessnanowires can be removed at this point with a chemical wash, which wouldrelease them from the substrate in regions where they are not pinneddown by the source and drain electrodes. Alternatively, excess nanowirescan be etched away using a traditional silicon etch.

At mobilities of 400 cm²/V·s and 200 cm²/V·s for, respectively,electrons and holes, the patterning resolution should be ˜5 um, with adesign gate (electrical) length of ˜10 μm, and sub-10 μm spaces betweenthe gate metal and the source and drain metal. Doping of the source anddrain can be self-aligned. Devices can be composed of approximately 1000nanowires aligned in parallel to provide adequate output current tocharge the interconnect lines and the gate capacitance of the nextstage. These interconnect lines can be as much as 25 μm wide and 1 cmlong and still achieve the desired rise and fall times. The overalldevice size can be approximately 250 μm wide and 10–50 μm long. Devicescan be grouped into cells providing low-level functionality to thedigital designer (inverters, NAND gates, etc.). These cells can be on anorder of 500 μm². This allows for generous wiring alleys and yet a 100transistor circuit will still be well under 1 cm².

The skilled artisan will appreciate that exact design rules will bedictated by the actual mobilities achieved, and could range to largerthan 20 μm for DION films with mobilities of 1500 cm²/V·s and 400cm²/V·s for, respectively, electrons and holes. Likewise, exact filmperformances will be used to define the lithographic designrequirements.

Exemplary CMOS TFT Device Fabrication Process

Described below is an exemplary fabrication process for DION TFTsfabricated from high-mobility III–V materials, e.g., to achieve amobility of greater than 1000 cm²/V·s, rather than via complex circuitdesign. Since this material within a mixed-composition DION thin-filmcan be use for RF signal processing, the discussion focuses on n-channeldevice designs.

FIGS. 26A and 26B are, respectively, a schematic and a layoutillustration of a static CMOS two-input NAND gate.

Synthesis of silicon nanowires can be further developed to achieveoptimal performance. This includes development of high-k (relativepermittivity) dielectrics shell materials and correspondingcircumferential gate shell deposition. III–V group high electronmobility materials can also be further developed, particularly withregards to an advanced core-shell structure for surface passivation,gate dielectrics, circumferential gates, and modulation doping.

Silicon nanowire growth can be optimized to further enhance performance.Higher mobility SiGe alloy materials can also be developed, if evenhigher carrier mobility is required. Specifically, Si_(1-x)Ge_(x) and Genanowires can be produced. An Au colloid catalyzed chemical vapor growthapproach with a mixture of SiH₄ and GeH₄ or GeH₄ as the reactant sourceand B₂H₆ or PH₃ as the dopant source can be used to grow Si_(1-x)Ge_(x)or Ge nanowires. Existing knowledge of the CVD process forSi_(1-x)Ge_(x) thin film and experiences with silicon nanowire growthcan be leveraged to optimize the growth conditions for and achieve highquality Si_(x)Ge_(1-x) or Ge nanowires. The resulting materials can befully characterized and carefully analyzed with various microscopic(e.g., SEM, TEM, etc.) techniques, and electrical transport propertiescan also be thoroughly tested and optimized.

A Si/SiO₂ core-shell structure for silicon nanowire surface passivationand gate dielectrics has already been developed. Either direct thermaloxidation or CVD deposition of SiO2 onto silicon nanowires can producethe shell structure. For Si_(1-x)Ge_(x) or Ge nanowires, this experiencein Si/SiO₂ core-shell structures can be exploited. In the case of SiGenanowires, core shell-structures can better be produced by CVDdepositing a pure SiO₂ shell, rather than direct thermal oxidation ofSi_(1-x)Ge_(x) nanowires. Direct oxidation of Si_(1-x)Ge_(x) nanowirewill likely produce a mixed SiO₂/GeO₂, which tends to have many trappingstates. To produce a shell of SiO₂ on SiGe nanowires, SiH₄ and GeH₄ willbe depleted in the reaction chamber after the nanowire growth isterminated. The furnace temperature is changed to the desiredtemperature for oxide deposition and a mixture of SiH₄ and O₂ isintroduced into the chamber to achieve oxide deposition. Thetemperature, SiH₄ and O₂ partial pressures, and time can be carefullycontrolled to deposit an oxide shell of desired thickness. The resultantmaterials can be thoroughly analyzed with a TEM. The electronicproperties of SiGe/SiO₂ nanowires can be tested in a FET structure. Fromthose cases in which the SiGe/SiO₂ interface is optimized forsuppressing interface trapping states, a SiGe/Si/SiO₂ core-shellstructure can be developed. In this situation, when SiGe nanowire growthis terminated, a shell of intrinsic silicon with a controlled thicknesscan be epitaxially deposited on the surface of SiGe nanowires. Lastly,an outer SiO₂ shell can be produced using the same approached developedin Si/SiO₂ core-shell system.

Characterization results of silicon nanowires suggest that facetedsilicon nanowires can be formed under controlled growth conditions. Thisknowledge can be leveraged to investigate epitaxial growth of a strainedSi_(1-x)Ge_(x) layer to further improve mobility. After termination ofsilicon nanowire growth, the condition of the reactor can be changed tosupport epitaxial growth of a thin layer of Si_(1-x)Ge_(x). This can befollowed by a thin layer of silicon, and finally of a layer of SiO₂ toyield a core shell structure with a silicon nanowire core, an activelayer of strained Si_(1-x)Ge_(x), a capping layer of silicon (to reducetraps), and a dielectric layer of SiO₂.

Thermal or laser vaporization can be used to produce III–V groupnanowires, such as from InP. InP nanowires synthesized this way can besufficient for testing and small scale device fabrication. However, thisprocess cannot be scaled to device implementation over large areasbecause of the intrinsic limitations of thermal and laser evaporationprocesses. A pilot production scale CVD process for InP nanowires(similar to that used for growth silicon nanowires) can be used toproduce nanowires at a wafer scale or larger. In this approach, InC1 ₃(trimethyl indium) and PH₃ can be used as the reactant source. SiH₄,H₂S, or H₂Se can be used as the dopant source. The growth temperature,partial pressure of each gas component, and overall base pressure can beadjusted to control the overall quality of the resulting nanowires. ThisCVD approach can also be explored for producing nanowires of other III–Vmaterials, such as InAs. The resulting nanowires can be fullycharacterized using approaches similar to those described above.

High performance III–V group materials are typically limited by surfacetrapping states. In order to eliminate such trapping states, variouscore-shell structures can be developed. It has been reported that CdS isan excellent capping layer for surface passivation of InP thin films.InP/CdS core-shell structures can be implemented using two approaches.First, at the end of the InP nanowire synthesis process, the substratetemperature is lowered in order to freeze the Au/InP eutectic droplet.Then, CdS can be evaporated to uniformly coat the InP nanowire surfaceto achieve an InP/CdS core-shell structure. Alternatively, the InP/CdSnanowires can also be produced in a solution phase by epitaxiallycoating CdS onto pre-synthesized InP nanowires. This approach has beensuccessfully implemented for InP/CdS quantum dots systems. The CdS shellin the InP/CdS structure can also be used as a gate dielectric. However,if the CdS shell is not of adequate quality for a gate insulator, anadditional dielectric layer of SiO₂ can be further deposited using anapproach similar to that described above.

To further improve the performance of nanowire-TFT devices, high-kdielectric materials can be used in the nanowire core-shell structure.High-k dielectrics (e.g., ZrO₂, HfO₂, etc.) have been actively pursuedto replace SiO₂ as gate insulators for silicon devices. High-k gateinsulators afford high capacitance without relying on ultra-small filmthickness. This allows for efficient charge formation in transistorchannels while reducing direct-tunneling leakage currents. To implementhigh-k materials in a nanowire core-shell structure, an AtomicLayer-by-layer Deposition (ALD) system is used to grow a ZrO₂ shell onthe surface of the chosen nanowires. A ZrCl₄ precursor and an H₂Ooxidizer in a high purity N₂ carrier gas can be used as the reactantsource. The deposition process can be carried out at a controlledtemperature and base pressure to ensure a high quality ZrO₂ thin film.The thickness of the ZrO₂ layer can be characterized by a TEM.

Exploiting the quantum electronic effect in small diameter nanowireswith a modulation-doped core-shell structure can produce higher electronmobility TFTs. In a manner analogous to the process of producingconventional two dimensional (2D) semiconductor superlattices and 2Delectron gas, a multi-core-shell nanowire structure can be produced toseparate the dopants from the active conducting channel to furtherenhance the carrier mobility. For example, ultra-high electron mobilitycan be realized in a structure comprising an intrinsic semiconductorcore (e.g., GaAs), an inner-shell of a thin spacing layer (an intrinsicmaterial of a larger bandgap, e.g., AlGaAs), and an outer-shell of adoping layer (a doped semiconductor, e.g., n-type AlGaAs). In this way,the dopants are only present in the outer shell material, separated fromthe active core, while the electrons can readily tunnel through thespacing layer into the active core material, which significantly reducesimpurity-related scattering in the core. In addition, when the diameterof the nanowire cores are smaller than a critical value (e.g., ˜20 nmfor GaAs), quantum mechanical phenomena can further suppress scatteringand lead to very high mobility values (e.g., theoretical calculationspredict mobility up to 10⁸ cm²/V·s for modulation-doped GaAs nanowires).To implement this approach, one type of nanowires is first grown usingan approach similar to that described above. The catalytic activity ofthe end Au droplet can be terminated by suddenly changing growthconditions. A second vapor phase can then be introduced with conditionscontrolled to cause uniform epitaxial growth on the nanowire surface andthereby produce a core-shell structure. Such a process can be repeatedmany times to produce a multi-core shell structure, if desired. Thedoping type of the core-shell structure can be flexibly changed andcontrolled to obtain desired properties. Lastly, the surface of thenanowires can be terminated with various gate dielectrics as describedabove.

In some cases, it may be desirable to provide a nanowire structure thatseparates the dopants from the active conducting channel. Such astructure comprises an intrinsic semiconductor core (e.g., GaAs), aninner-shell of a thin spacing layer (an intrinsic material of a largerbandgap, e.g., AlGaAs), and an outer shell of a doping layer (a dopedsemiconductor, e.g., n-type AlGaAs). By separating the dopants from theactive conducting channel (core) and exploiting the quantum-confinementeffect, a very high carrier mobility can be realized. To further enhancethe device performance of these new nanowires the surface can be coatedwith a conducting shell to surrounding the gate dielectric as acircumferential gate. For example, a doped amorphous or polysiliconshell can be deposited onto any nanowire surface in a way similar tothat developed for Si/SiO₂/p⁺Si core shell nanowires as described above.Similarly, the surface of the nanowires can be coated with a thin metallayer by, for example, electro-less metal deposition.

The product of each synthetic step can be carefully studied with opticalmicroscopy, SEM, or TEM to analyze the morphology, diameter, or lengthof the nanowires. EDX can be used to assess the chemical composition ofthe nanowires. The information can further be used as feedback tosynthesis to optimize the overall procedure and achieve precise controlof the material parameters.

Direct doping of nanowires during synthesis has been described withregards to including a dopant precursor gas in a reactant mixture andmodulated doping by coating undoped wires with a shell containing adopant. For these doped nanowires, a direct metal contact can be usedfor source and drain electrodes. However, in some cases, the metalcontact may not be good enough for low-doped materials or intrinsicmaterials (particularly for devices that operate in the inversion mode).To this end, contact doping is another alternative. Since nanowires havea very small diameter (˜20 nm) and a very high surface to volume ratio,a quick contact with a precursor and an extremely short diffusion can besufficient to drive enough dopants into the nanowires. In general, theapproach includes steps of surface cleaning, chemical absorption of adopant precursor onto the nanowire surface, and a quick supply of theenergy to drive the dopant into the nanowires. Standard surface cleaningtechniques, such as wet etching, plasma etching, and heating underultrahigh vacuum (to remove native oxides from the silicon nanowiresurface) can be used. The precursor can be a gaseous material (e.g., PH₃for n-doping silicon wires), and in situ generated species (e.g., ap-dopant generated by a heavily doped p-Si layer). The energy source canbe a resistive heating source, such as rapid thermal processing (RTP)lamps, or a focused laser. Laser heating can also be used for contactdoping on plastic substrates.

The fluidic flow approach can also be implemented in a different andmore scalable version. FIG. 27 schematically illustrates a system forroll-to-roll compatible flow-based DION film deposition. The apparatuscan comprise an inclined surface for mounting the substrate, a spray-barparallel to the substrate, and a pump system to produce flow through thespray-bar. Solution is sprayed onto the surface of the substrate whilethe substrate is continuously moved upwards by a motor in a roll-to-rollprocess. Downward flowing solution can orient nanowires in the flowdirection. Further alignment can be induced by rubbing the nanowireswith a micro scale brush similar to the manner in which liquid crystalis oriented in an LCD manufacturing process. The motion of the substrateand the substrate inclination are tailored to produce optimum wireuniformity and density. If a very high density deposition is required,the process can be repeated. The nanowires that are not deposited remainin solution, undamaged, and can be recycled. Nanowire concentration inthe effluent can be monitored and solvent can be added or removed (byevaporation) as needed. If necessary after deposition is complete, thesubstrate mounting surface can be heated to ensure rapid solventevaporation. A second spray-bar can also be added to the apparatus. Thissecond spray bar can spray pure solvent to wash away all undepositednanowires. This can minimize the deposition, as the solution evaporates,of randomly oriented nanowires. An optimum surface chemistry of thenanowires and the substrate, as well as the deposition parameters(solvent viscosity and volatility, nanowire concentration, substrateinclination angle, spray rate, and spray bar motion profile) can bedetermined through an iterative process.

In certain alternative aspects, a Langmuir-Blodgett film approach can beadopted for larger scale nanowire deposition. This can provide uniformalignment over very large areas. However, the results from fluidic flowalignment can still be used for initial test of device fabrication andcharacterization. In order to achieve uniform alignment over largeareas, a large scale assembly approach can be developed based onLangmuir-Blodgett (LB) films. Langmuir-Blodgett alignment has been usedto form thin films of nanoparticles and to align nanorods. This approachcan be extended to the alignment of nanowires through the incorporationof appropriate surface chemistry (as described above) to produce anoriented nanowire thin film. In this approach, the nanowires can firstbe functionalized and suspended in non-polar solvent. Such a non-polarnanowire suspension can then be transferred onto the surface of thewater in an LB trough. At sufficiently low-densities, the nanowires forman isotropic distribution with random orientation. As the surface iscompressed in one dimension, however, it becomes increasingly difficultfor the nanowires to remain pointed in random directions. The nanowirescan undergo a transition to a more ordered anisotropic phase withuniaxial symmetry, particularly a nematic or smectic phase.

This transition has been observed both in Monte-Carlo simulations and inreal experiments in the case of alignment of thin film nanorods (havingan aspect ration <10). Although the larger aspect ratio of nanowireswould suggest a greater ability to align by LB, there is a risk that thelength of nanowires could cause problems because they would not be ableto rotate as freely to reach their lowest energy state within the film(the equivalent of a local minima). In this case, mild agitation can beincorporated to help prevent films from forming at these high-energyminima so that eventually the lowest energy aligned state is reached. Inaddition, directional capillary forces and van der Waals attractionsbetween nanowires can be used to further enhance the parallel alignmentof the nanowires and the formation of an oriented nanowire thin film.This issue can also be addressed by inducing some pre-alignment prior tosurface compression. A number of strategies can also be employed toachieve this goal. For example, a flow process can be combined toachieve some pre-alignment. An electrical field can also be applied toenhance alignment of the wires.

Once aligned, the film can be further compressed to increase the degreeof orientation. Then the film can be transferred onto a desiredsubstrate. In addition to the rate at which the substrate is removedfrom the LB trough, the nanowire density can be controlled by the ratioof surfactants and nanowires and by the amount of surface compression.Different transfer protocols can be developed to avoid disturbing thealignment during the transferring process. The surface coverage can becharacterized using an approach similar to that described above.

In other respects, printing technologies can be employed that arecompatible with a large volume roll-to-roll process and that can beintegrated with the overall device fabrication process. For example, acontact printing approach can include the following. First, with aprinting ribbon continuously running through a nanowire solution at acontrolled speed, the shear flow near the ribbon surface can align thenanowires along one direction. The surface chemistry of the nanowiresand the substrate, as well as the time of duration of the ribbon in thesolution, can be controlled to achieve a desired nanowire density on theribbon. After passing through the nanowire solution, the ribbon cancontinue to move across the desired device substrate. Computercontrolled motion van bring the ribbon and the substrate into contact.By this shearing method, the aligned nanowires can be transferred ontothe substrate by controlling the electrostatic interactions or thecomplementary chemical interactions. This method can be programmed todirectly print aligned nanowire thin film onto the substrate in adesired pattern. The substrate can be continuously fed though the systemin a roll-to-roll fashion.

Various approaches can be employed for mixed nanowire thin filmdeposition. These include, e.g., a “multiple-layer” fashion and a“stripped or check-board pattern” format. To achieve a multiple layermixed nanowire thin film, any of the previous approaches can be used toform a first layer of nanowire thin film. This can be followed by devicefabrication on the first layer. Then a planarizing insulating layer canbe deposited. This can be a polymeric material, such as SU8 resist.Next, a second layer of a different type of nanowires is aligned ontothe surface of the insulating layer. Again, this can be followed bydevice fabrication on the second layer. In this “multi-layer” fashion,since each layer is nearly independent from the other layers, all wiredeposition and device fabrication processes for each layer can becarried out using the technologies described above. Inter-layerelectrical connection can be done by photolithographically patterningthe intermediate insulating layer to have open windows in it. This canbe followed by metallization to form an inter-layer connection.

Stripped patterns of mixed nanowire thin film can be formed in a numberof different ways:

A mixed nanowire thin film can be obtained by using multiple parallelchannel flows such that alternating channels have different nanowiresolutions flowing through them. In this approach, a mixed nanowire thinfilm can be obtained in a single flow process with resolution of about˜10-μm.

A mixed nanowire thin film can also be obtained with a successiveelectrostatic assembly process. In this process, the substrate is firstpatterned with a pattern array of electrodes. These can be used to applyan electric field for electrostatic assembly. To achieve a mixed film,the substrate is processed multiple times with different electrodesenergized.

A mixed nanowire thin film can also be obtained by first patterningselected regions of the substrate surface with different chemical orbiological functionalities. Two types of nanowires could befunctionalized with different chemical/biological groups, each of whichis complementary to a particular surface function on the selected areaof the substrate surface. Two different nanowires could then be exposedon the substrate to achieve a mixed nanowire thin film with twodifferent types of nanowires on different areas of the substratesurface.

A mixed nanowire thin film can also be obtained using a multiple stepcontact printing approach. In this approach, a patterned stamp is firstmade from an elastic conformal material (e.g., PDMS). The first type ofnanowires is then assembled onto the stamp surface. The nanowires couldthen be stamped onto the device substrate to obtain the first type ofnanowire thin film on a selected area of the substrate. This process canthen be repeated to apply a different type of nanowire thin film on adifferent region of the substrate surface.

The mixed nanowire thin film can also be formed by modifying theprinting approach, developed for the single composition nanowire thinfilm deposition process, to include a “color” printing technology.Different nanowires are considered analogous to different colors in aconventional color printer. For example, for the contact printingtechnology described above, the substrate is successively run belowmultiple ribbons with different nanowires to achieve a mixed nanowirethin film with a computer controlled pattern. This process is compatiblewith a roll-to-roll process.

The present invention can lead to significant increases in bothcomplexity (greater than one thousand devices) and performance (at least100 MHz clock rates). These improvements can be accompanied bysubstantial improvements in CMOS mobilities, with exemplary values forelectrons and holes in an inversion-mode device of, respectively, 1000cm²/V·s and 400 cm²/V·s. For electrons and holes in a Ge/Si/SiO2core-shell-shell structure with an intrinsic germanium conductingchannel and a doped silicon shell to supply carriers, performance couldbe as high as, respectively, 3000 cm²/V·s and 1500 cm²/V·s. The skilledartisan will appreciate that: (1) inversion-mode devices can be producedto reduce threshold voltage, (2) source and drain doping can beself-aligned to reduce output impedance, and (3) devices can be producedto have lower threshold voltages, lower power supply voltages, andreduced power dissipation.

Example Applications of Nanowire Films of the Present Invention

Numerous electronic devices and systems can incorporate semiconductordevices that use thin films of nanowires, according to embodiments ofthe present invention. Some example applications for the presentinvention are described below for illustrative purposes. The presentinvention is not limited to these applications. The applicationsdescribed herein can include aligned or non-aligned thin films ofnanowires, composite or non-composite thin films of nanowires, and caninclude any other nanowire or nanowire film variation(s) describedabove.

Lightweight Distributed Sensor Networks for Perimeter Security

An initial application is described below, namely, lightweightdistributed sensor networks for perimeter security. The system cancomprise multiple units of integrated macroelectronic circuitscontaining sensors, logic, and RF communications that are printed onlightweight, flexible plastic substrates. When distributed around asecure perimeter, each of these units can monitor its local environmentfor one or more stimuli. Upon detection of a specific signal, theinformation can be communicated back to a base station throughindividual sensor elements.

The basic concept is to fabricate a multi-functional monolithic deviceonto a lightweight flexible substrate that is capable of sensing one ormore aspects of its environment, electronically processing thatinformation, and transmitting it back to a base station. One or morespecific sensors can be developed. The present invention can support alarge variety of different sensor types. In particular, DION-basedsensors can be fabricated as motion sensors, light sensors, soundsensors, etc. With the technology of the present invention, alightweight and low-cost distributed sensor unit that can be dispersedaround a secure perimeter to monitor for a variety of signals. FIG. 28schematically illustrates the concept of a distributed sensor network ofthe present invention. The figure shows the components of such as systemand how it would operate in a practical application.

The form-factor and cost-factor of each sensor unit enable thisapplication. By fabricating the devices on a flexible plastic substrate,a sensor can be camouflaged (i.e., the sensor can be made to look likesomething that would normally be found in its surroundings, such as aleaf on a tree). By fabricating the devices at a low cost, a largenumber of sensors will be able to be deployed in a cost-viable manner tosecure a location. This effectively realizes a low-cost network ofindependent sensors. In a preferred aspect, the low-cost electronicsubstrates can be printed onto a single lightweight flexible substrate.

This application extends the concept of “high-performance electronicsdistributed over a large area” to include electronics that: (1) aredistributed even beyond a single substrate and (2) incorporateelectrical and non-electrical interconnections between functionalelements within a “mega-electronics” integrated system (i.e., large than“macroelectronics”). The technology for high-performancemacroelectronics is particularly critical for a practical realization ofthis application. First, the cost of production should be low to allowsensor nodes to be distributed over large areas around a secureperimeter. Second, the flexible substrate allows the device to becamouflaged (e.g., cut and painted to look like a leaf, a gum wrapper,etc. that would go unnoticed in the area in which it is deposited). Thisapproach to this type of application is very different from conventionalapproaches. In the past, efforts have been made to make the tags verysmall so that they are hard to see. Unfortunately, small size alsoreduces the size of the antenna, which not only reduces read range, butalso dramatically complicates the design of the system. The uniqueform-factors enabled by DION electronics can make the sensors hard todetect not merely because they are small, but more specifically becausethey blend in with their environment. In addition, this technology canalso be used to fabricate other unique form-factors, such as “smart”wallpaper, smart paper, smart folders, or smart anything that does notlook like a sensing tag.

The distributed sensor network of the present invention can be realizedby production of: (1) specific printable sensor materials, (2) printableantenna-designs compatible with the network requirements, (3) softwareand hardware to process the incoming information, and (4) specificmulti-functional nanowire thin-film circuits capable of sensor, RF, andelectonic signal processing.

In an embodiment, the distributed sensor network comprises a pluralityof passive RF-sensor elements (e.g., “sensing tags”), complemented witha small number of active beacons around the secure perimeter. The activebeacons provide RF power to and query the passive units and coordinatethe flow of data to the base station. Optionally, the base station couldbe connected via internet or satellite to a central commandheadquarters, which could be monitoring numerous such sensor networksworldwide. FIG. 29 shows a schematic illustration of an RFID/Sensor tagsystem. The figure illustrates a basic circuit design of an RFID/Sensortag system, based upon the nanowire based substrates described herein.

The circuits can receive sensor output and convert it to digital datafor input to the RFID tag circuit. The output from the sensor typicallyis an analog signal, which is converted to a digital bit stream at adesired degree of resolution. For certain applications, the mere factthat a sensor has sensed something is sufficient. In such an applicationa single bit value of zero or one can be satisfactory. In anotherapplication, a 32 bit digital output could be required so that ananalog-to-digital circuit can be placed at the “front end” of the tag ICand will interface with one tag IC input pad connected to the sensor.These configurations can be replicated for multiple sensors.

The circuit can receive the output of the sensor circuit and append thedigital data to the tag ID memory register. This latter circuit can beplaced at the “back end” of the tag ID so that it receives the digitaloutput of the “front end” circuit and loads the data into the ID outputregister. The tag ID can be configured in sections such that the frontpart comprises a unique tag ID number, which uniquely identifies thetag, and back part includes the sensor data. If the sensor is notdetecting anything, the back part will contain a string of 32 zeroes.Once the sensor has detected something, its output will be sent outalong with the tag ID number. In this way, the reader will be able todetermine both a sensor event and the sensor location by means of thetag ID number.

In another embodiment, the distributed sensor network comprises acollection of active RF-sensor elements powered by thin-film batteriesor photovoltaic nanowire materials integrated into the multi-compositionDION thin film.

This application is only one of many potential applications that can beenabled by the DION thin-film technology. In particular, theperformance, cost, and ability to fabricate multifunctional electronicelements can be especially valuable in the development of systems suchas lightweight, portable X-ray imagers for security applications, phasedarray antennas for wireless communications, radar scanners for securityapplications, for flexible displays, for lightweight, space-constrainedelectronics, and numerous other applications.

Beam-Steering of Antennas in RFID Tags

According to an application of the present invention, thin films ofnanowires or nanotubes are used in radio frequency identification (RFID)tags and/or RFID tag readers to provide enhanced performancecharacteristics. The nanowire films of the present invention describedabove enable the use of beam-steering arrays in tags and/or readers.Such enhanced RFID tags can use the power of an incoming RF signal totune the phase of the array to maximize the power of the incoming signal(i.e., effectively pointing the antenna of the tag toward the reader).Furthermore, the enhanced RFID tags are low-cost, and can receive asignal efficiently at any orientation. By focusing an antenna of a tagtoward the reader, the signal transmitted by the tag is concentrated,dramatically increasing the range over which the tag can be detected. Aresult is that less energy is lost due to isotropic signal transmission,because the tag directs its transmitted signal toward the reader.

Furthermore, as a result of these enhancements, tags can receive andprocess information more quickly than traditional tags, dramaticallyincreasing the read rate of a group of tags by a reader. The combinationof a macro-electronic substrate with the steerable antenna in an RFIDtag allows the RFID tag to communicate at any orientation relative tothe reader, and from greater distances from the reader (>100 meters)than conventionally possible. Furthermore, the present invention isapplicable to passive and active RFID tag types.

A reader can incorporate a nanowire-enabled beam-steering array toprovide similar performance enhancements. A beam-steering arrayincorporated into a reader can focus an antenna of the reader towards atransmitting tag to detect the tag at any orientation relative to thereader, and to increase the range over which the tag can be detected. Asa result of this, the reader will be able to process greater numbers oftags more quickly. Furthermore, a major reduction in interference fromnearby tags can be achieved. This is because tags tend to be located indifferent locations, and the antenna will tend to be focused on fewertags, such as a single tag, at any one time.

The following subsection describes an example RFID tag and readerenvironment incorporating a nanowire-enabled beam-steering array of thepresent invention. The subsequent subsection provides further detail ofnanowire-enabled beam-steering arrays, followed by a subsectiondescribing nanowire-enabled adjustable phase delay embodiments that, canbe incorporated into a beam-steering array and other devices.

RFID Tag and Reader Embodiments Incorporating a Nanowire-EnabledBeam-Steering Array

FIG. 30 illustrates a RFID communications environment 3000, according toan example embodiment of the present invention. In environment 3000,reader 3002 communicates with one or more tags 3004, shown for examplein FIG. 30 as tags 3004 a–3004 c. While three tags 3004 a–3004 c areshown in FIG. 30 for illustrative purposes, environment 3000 can includeany number of tags 3004, including hundreds, thousands, and even greaternumbers.

Tags 3004 are typically affixed to items that are to be monitored. Thepresence of a tag 3004, and therefore the presence of the item or objectto which the tag 3004 is affixed, can be checked and monitored by reader3002. Reader 3004 monitors the existence and location of the itemshaving tags 3004 affixed thereto through wireless interrogations.Typically, each tag 3004 has a unique identification number that reader3002 uses to identify the particular tag 3004 and the respective item.

For example, as shown in FIG. 30, reader 3002 transmits an interrogation3006 to a group of tags 3004, typically at a radio frequency.Interrogation 3006 is received by one or more tags of the group, such asby tags 3004 a–3004 c. Each of tags 3004 a–3004 c individually processthe received interrogation 3006, and may respond if appropriate. Asshown in FIG. 30, tags 3004 a–3004 c can transmit respective responses3008 a–3008 c. Readers 3002 and tags 3004 can communicate according to avariety of protocols that would be known by persons skilled in therelevant art(s).

FIGS. 31A and 31B illustrate block diagrams showing detailed exampleconfigurations for tag 3004 and reader 3002, respectively, according toembodiments of the present invention. The configurations of tag 3004 andreader 3002 shown in FIGS. 31A and 31B are provided for illustrativepurposes. The present invention is applicable to other tag and readerconfigurations, and other communication environments, as would beunderstood by persons skilled in the relevant art(s). For example, inone environment, tags 3004 include beam-steering functionality, whilereaders 3002 do not. In another environment, tags 3004 do not includebeam-steering functionality, while readers 3002 do include beam-steeringfunctionality. In still another environment, both tags 3004 and readers3002 include beam-steering functionality.

As shown in FIG. 31A, tag 3004 includes an antenna 3102, a transceiver3104, a storage 3106, a beam-steering array 3108, and a tag controller3110. Tag 3004 further includes a substrate 3118 on which thesecomponents are mounted, attached, printed, or otherwise formed. Thecomponents of tag 3004 can include any electronic hardware, software,and/or firmware as necessary. Note that as is further described below,in an alternative embodiment, beam-steering array 3108 can perform thefunctions of antenna 3102. Thus, in such an embodiment, antenna 3102 maynot be necessary.

Antenna 3102 is used to receive and transmit EM signals, such asinterrogation 3006 and response 3008 a, respectively. Antenna 3102 canbe any type of antenna appropriate for use in an RFID tag.

Transceiver 3104 is coupled to antenna 3102. Transceiver 3104 can be anytype of transceiver, or combination of receiver and transmitterappropriate for use in an RFID tag. Transceiver 3104 performs frequencydown-conversion and/or de-modulation of an EM signal received by antenna3102, as needed, and outputs an information signal 3114 to tagcontroller 3110. Furthermore, transceiver 3104 receives informationsignal 3114 from tag controller 3110, and performs modulation and/orfrequency up-conversion of information signal 3114 as required by RFIDtag 3004. The up-converted signal is transmitted by antenna 3102.

Tag controller 3110 controls operation of tag 3004. Tag controller 3110can include any hardware, software, firmware, or any combination thereofnecessary to perform its functions. For example, tag controller 3110 andstorage 3106 can be present in an application specific integratedcircuit (ASIC). Tag controller 3110 processes information signal 3114when received from transceiver 3104. For example, tag controller 3110processes information signal 3114 to determine whether a receivedinterrogation 3006 is directed at the respective tag 3004, and generatesan appropriate response. Tag controller 3110 outputs the generatedresponse to transceiver 3104.

Storage 3106 can store information related to tag 3004, including anidentification number. Tag controller 3110 accesses storage 3106 todetermine the stored information. Tag controller 3110 can use the storedidentification number to determine whether an interrogation 3006 isdirected to the respective tag 3004. Storage 3106 can be read-onlystorage (e.g., a read-only memory (ROM) device), or can also bewrite-capable for storing additional information.

As shown in FIG. 31A, tag 3004 includes beam-steering array 3108. In anembodiment, an EM signal transmitted from antenna 3102 meets,encounters, impinges upon, or is otherwise received by beam-steeringarray 3108. For example, antenna 3102 can transmit the EM signaldirectly toward beam-steering array 3108, or antenna 3102 canisotropically transmit the EM signal to reach beam-steering array 3108.Beam-steering array 3108 re-directs the EM signal as directed by tagcontroller 3110.

Note that in an alternative embodiment, as is further described below,beam-steering array 3108 additionally performs the function of antenna3102. In such an embodiment, antenna 3102 is not necessary. Thus, asshown in FIG. 31A, transceiver 3104 can optionally be directly coupledto beam steering array 3108 via link 3120, and beam-steering array 3108transmits the EM signal directly.

Tag controller 3110 includes an array controller 3112 portion thatcontrols beam-steering array 3108. Array controller 3112 generates anarray control signal 3116 that is received by beam-steering array 3108.Array control signal 3116 can be a serial signal, or a parallel signalbus. Array control signal 3116 controls a plurality of phase-adjustmentelements of beam-steering array 3108 to control re-direction of the EMsignal transmitted by antenna 3102, or control direction of the EMsignal transmitted by an array of antenna elements of beam-steeringarray 3108. Array controller 3112 can cause beam-steering array 3108 todirect or re-direct the EM signal by directing the EM signal in anydirection, by focusing the EM signal, by spreading the EM signal, and byperforming any combination thereof.

In a similar fashion, when tag 3004 is in a receiving mode,beam-steering array 3108 can direct its array of antenna elementstowards an incoming EM signal. In an embodiment, array controller 3112controls the plurality of phase-adjustment elements of beam-steeringarray 3108 to receive or to control re-direction of the EM signaltransmitted by reader 3002. For example, beam-steering array 3108 canre-direct an EM signal received from reader 3002 at any directiontowards antenna 3102, or can directly receive the EM signal from reader3002.

In an embodiment, array controller 3112 includes a scanning algorithm.The scanning algorithm can be used to scan for an optimal direction inwhich the EM signal broadcast by reader 3002 is strongest, in effect,directing beam-steering array 3108 toward the reader 3002. Thus, signalreception by tag 3004 is improved. Such scanning algorithms will beapparent to persons skilled in the relevant art(s) from the teachingsherein. Once this optimal direction is determined, tag 3004 can directits resulting response 3008 toward reader 3002. Thus, the response bytag 3004 is more likely to be received by reader 3002, and tag 3004 cantransmit from further distances because the transmitted response 3008 isconcentrated.

FIG. 31B illustrates a block diagram for reader 3002, according to anexample embodiment of the present invention. As shown in FIG. 31B,reader 3002 includes an antenna 3122, a transceiver 3124, beam-steeringarray 3108, and reader controller 3130. These components of reader 3002have a similar function to the corresponding components of tag 3004.Transceiver 3124 is coupled to reader controller 3130 by an informationsignal 3134. Beam-steering array 3108 is coupled to reader controller3130 by an array control signal 3136. Note that reader 3002 can be ahandheld or non-handheld unit. Furthermore, in an embodiment, reader3002 is coupled to a network or computer system by a wireless or wiredcommunication link 3150.

Reader controller 3130 includes an array controller 3132, which issimilar to array controller 3112 of tag 3004. In a transmit mode forreader 3002, array controller 3132 controls a plurality ofphase-adjustment elements of beam-steering array 3108 to controlre-direction of an EM signal transmitted by antenna 3122. For example,the EM signal can be directed towards a particular tag 3004, or group oftags 3004. Furthermore, when reader 3002 is in a receive mode, arraycontroller 3132 controls the plurality of phase-adjustment elements ofbeam-steering array 3108 in reader 3002 to re-direct an EM signal beingreceived from a tag 3004 towards antenna 3122 of reader 3002.

Alternatively, as described above for tag 3004, beam-steering array 3108of reader 3002 additionally performs the function of antenna 3122, sothat antenna 3122 is not required to be present. Array controller 3132can control the plurality of phase-adjustment elements of beam-steeringarray 3108 to control a transmit and a receive direction for theplurality of antenna elements. Thus, as shown in FIG. 31B, transceiver3124 can optionally be directly coupled to beam steering array 3108 vialink 3140.

Note that in an embodiment, array controller 3132 can include a scanningalgorithm similar to that of array controller 3112 (shown in FIG. 31A).For example, the scanning algorithm can be used to find the direction inwhich an EM signal broadcast by a tag 3004 is strongest, in effect,directing beam-steering array 3108 of reader 3002 toward thebroadcasting tag 3004.

A variety of embodiments for beam-steering array 3108 are possible, andare within the scope and spirit of the present invention. Furthermore,in embodiments, beam-steering array 3108 incorporates thin films ofnanowires. The thin films of nanowires enable phase adjustmentfunctionality of beam-steering array 3108. The following subsectionsdescribe example detailed embodiments for beam-steering array 3108.

Beam-Steering Array Embodiments

Embodiments for beam-steering array 3108 are described in this section.As described above, beam-steering array 3108 allows directing an EMsignal in any any direction, focusing the EM signal, spreading the EMsignal, and any combination thereof. The thin films of nanowires of thepresent invention are incorporated into beam-steering array 3108 toenable phase adjustment functionality, as further described below. Notethat the present invention is applicable to all types of beam-steeringarrays, including reflector types and multi-antenna element array types.For illustrative purposes, reflector types and multi-antenna elementarray types are described in the subsections below. However, it is to beunderstood that the present invention is also applicable to other typesof beam-steering arrays.

Reflector-Type Beam-Steering Array Embodiments

This subsection describes example beam-steering arrays that operate as abeam reflector. Although the following description shows particularreflector configurations for illustrative purposes, it is to beunderstood that the nanowire technology described herein is applicableto any type of beam reflector, and such beam reflectors are within thescope and spirit of the present invention.

FIG. 57 shows an example beam-steering reflector 5700 operating in atransmitting mode. An antenna 5702 (shown as a horn antenna forillustrative purposes) transmits an EM signal 5704 that arrives atbeam-steering reflector 5700. Beam-steering reflector 5700 re-directs EMsignal 5704, shown as re-directed EM signal 5706. Note that in areceiving mode, beam-steering reflector 5700 operates in an analogousmanner.

Beam-steering reflector 5700 shown in FIG. 57 is a reflector-typevariation of beam-steering array 3108, and is based on the concept offrequency-selective surfaces (FSSs). Tunable FSSs are the basis forbeam-steering reflector 5700. FSSs have the ability to reflect RFsignals with a predetermined phase (φ). Furthermore, an FSS can befabricated in thin printed-circuit-like systems. By configuring the FSSto be tunable, the phase φ of the FSS can be con-trolled. For example,beam-steering reflector 5700 includes a plurality of cells forming asubstantially flat surface 5710. By configuring each cell of surface5710 to be controlled independently, φ profiles, or contours, can beimparted onto surface 5710 of beam-steering reflector 5700. As such, asubstantially flat surface 5710 of beam-steering reflector 5700 can havecharacteristics similar to a parabolic reflector, without thedisadvantage of the space required by a three-dimensional (3D) surface.Surface 5710 can further be tuned or adjusted, making beam-steeringreflector 5700 steerable. Furthermore, beam-steering reflector 5700 doesnot require moving parts to be steerable.

Sheets of a material displaying the properties of a perfect magneticconductor (PMC) can be used as an FSS to enable beam-steering reflector5700. Such PMC surfaces can include doubly periodic, resonant electricalLC (inductance-capacitance) circuits, the behavior of which at resonanceapproximates that of a theoretical PMC. Thus, such PMC surfaces haveproperties are frequency-dependent, and are therefore suitable as a FSS.

Tunable-phase PMC surfaces have an embedded inductance L or capacitanceC that is electrically adjustable. Based upon conventional assemblytechniques, these surfaces allow electrical manipulation of thesurface's resonant frequency (f_(r)) and hence adjustment of thesurface's φ. Due to the cost and limitations of the presentmanufacturing techniques, these surfaces have been very limited in size,only 3″×6″(7.6×15.2 cm), and have been suitable only as conceptdemonstrations in on-the-bench tests. The macroelectronics of thepresent invention, including thin films of nanowires on large areasubstrates, allow the fabrication of useful sizes of tunable FSSreflector for beam-steering reflector 5700.

FIG. 33 shows a view of surface 5710 of an example beam-steeringreflector 5700. As shown in FIG. 33, beam-steering reflector 5700includes a plurality of cells 3302, including cells 3302 a–3302 c. Cells3302 of beam-steering reflector 5700 are each resonant. Furthermore,according to the present invention, each resonant cell of the pluralityof cells 3302 of beam-steering reflector 5700 can be configured to beindividually tunable. Hence, different areas of surface 5710 ofbeam-steering reflector 5700 can be made to have different reflectionphases. By configuring beam-steering reflector 5700 to have anon-uniform reflection phase characteristic, a reflected beam can befocused, spread, or steered as desired.

FIG. 34 shows a cross-sectional view of an example, fixed-frequency PMCstructure 3400. Three cells 3302 a–3302 c are shown in the example ofFIG. 34. FIG. 35 shows a perspective view of a portion of PMC structure3400, having a 2×2 array of cells 3302 a, 3302 b, 3302 d, and 3302 e. Asshown in FIG. 34, PMC structure 3400 includes a first electricallyconductive layer 3402, a second electrically conductive layer 3404, anda dielectric layer 3406. Dielectric layer 3406 is positioned betweenfirst and second electrically conductive layers 3402 and 3404.

First and second electrically conductive layers 3402 and 3404 can be anyelectrically conductive material, including a metal such as copper oraluminum, or a combination of metals/alloy. Dielectric layer 3406 can beany electrical insulator, and can be selected to enhance an inductanceand/or capacitance characteristic of a cell 3302. Second electricallyconductive layer 3404 is typically coupled to a ground or otherreference potential. PMC structure 3400 can be fabricated using standardtwo-layer printed-circuit-board (PCB) manufacturing techniques, forexample.

As shown in FIG. 34, each cell 3302 includes a “patch” or portion 3410of first electrically conductive layer 3402 that is separate from otherportions 3410. As shown in FIG. 35, portions 3410 can be substantiallyrectangular, although they can have other shapes, alternatively.Furthermore, each cell 3302 includes an electrically conductive via 3412formed therethrough that electrically couples the respective portion3410 to second electrically conductive layer 3404. Each portion 3410 andcorresponding via 3412 forms a thumbtack-like structure on secondelectrically conductive layer 3404.

FIG. 36 illustrates an inductance and capacitance involved in aresonance of cells 3302 a and 3302 b of a portion of PMC structure 3400.The resonance within PMC structure 3400 allows for the beam-steeringfunction of beam-steering reflector 5700. The resonance occurs becauseof a capacitance, C, and an inductance, L. Capacitance C exists betweeneach portion 3410 and the ground plane (or other reference potential) ofelectrically conductive layer 3404. Inductance L exists due to eachportion 3410, a corresponding via 3412, and the ground-plane (or otherreference potential plane) of second electrically conductive layer 3404.Thus, capacitance C and inductance L contribute to a resonant frequency,f_(r), of a cell 3302. An incident EM signal wave impinging onfixed-frequency PMC structure 3400 will be reflected in-phase if it hasa frequency of the resonant frequency, f_(r), of the FSS. An incident EMsignal wave impinging on PMC structure 3400 with frequency f will bereflected with varying amounts of φ, if it is off-frequency (f≠f_(r)),approaching 180° if far from resonance frequency f_(r)(f<<f_(r) orf>>f_(r)). For illustrative purposes, FIG. 37 shows a schematic view ofa transmission-line equivalent model 3700 of any one pair of cells 3302of fixed-frequency PMC structure 3400.

Using microwave analysis techniques, it can be shown how PMC structure3400 shown in FIG. 36 displays the properties of a PMC, that is, havingφ=0°. FIG. 40 shows a plot 4000 of a curve 4010 representing φ(f), orreflection coefficient phase versus frequency. Curve 4010 shows valuesfor φ ranging between ±180°. An example PMC operating range 4002 for φbetween ±90° is also shown (although the present invention can haveother ranges for φ). As shown in FIG. 40, range 4002 is relativelynarrow. By adjusting the resonant frequency f_(r) of a cell 3302, curve4010 can be made to “slide” left or right. This causes an operatingpoint (i.e., a φ value at a particular operating frequency) to moveupwards, i.e., towards +90°, or downwards, i.e., towards −90°, in phase,thus providing phase control.

To accomplish phase control in this manner, and to create aphase-adjustable and phase-conformal surface, either the capacitor valueC, the inductor value L, or both C and L must be adjusted. For example,these values can be adjusted electronically. To accomplish this, largeareas are required in the beam-steering reflectors to mount the requiredelectrical components. Furthermore, high electronic performance isrequired for the necessary RF processing. The large-area macroelectronicsubstrate of the present invention, incorporating films of nanowires,provides these capabilities.

PMC structure 3400 can be configured to be tunable, according to thepresent invention, to form beam-steering reflector 5700. To configurePMC structure 3400 as beam-steering reflector 5700, the resonantfrequency f_(r)(and hence the φ at a particular f) of the FSS cells 3302of PMC structure 3400 must be configured to be actively controllable. Inan embodiment, analog or continuous phase-adjustment of cells 3302 canbe accomplished by incorporating active loads, such as variable-Cvaractor diodes or L transformations of tunable elements throughtransistors. In another embodiment, discrete phase-adjustment of cells3302 can be accomplished by switching reactive components having variousvalues in and out of the resonant circuit of cells 3302.

FIG. 38 shows a cross-sectional portion of a beam-steering reflector5700, where active phase-adjustment elements are coupled to a PMCstructure to provide discrete tunability, according to an exampleembodiment of the present invention. In the embodiment of FIG. 38, atunable beam-steering array structure is created by coupling a circuitto each cell that can be discretely tuned with various L values (and/orC values). As shown in FIG. 38, cells 3302 are similar to those shown inFIG. 34. However, in FIG. 38, each cell 3302 includes an electricallyconductive via 3802 having a first end coupled to portion 3410, and asecond end 3804 extending through an opening 3806 in second electricallyconductive layer 3404. Furthermore, for each cell 3302, a pair ofinductors 3810 a and 3810 b are coupled between second end 3804 of via3802 and second conductive layer 3404 through a corresponding switch3830 a and 3830 b, respectively. Inductors 3810 operate asphase-adjustment elements for cells 3302. Selective control of switches3830 allow inductors 3810 to be discretely coupled to cells 3302 toprovide adjustment of the resonant frequency of cells 3302, thusproviding tunability to beam-steering reflector 5700. FIG. 39 shows aschematic view of a transmission-line equivalent model 3900 of any onepair of cells 3302 of beam-steering reflector 5700. Note that FIG. 39shows an adjustable inductor L, which can be adjusted by switching indifferent valued inductors, or by varying a length of an inductor, or byother techniques described elsewhere herein, or otherwise known.

Note that although the example of FIG. 38 shows two inductors 3810 a and3810 b for each cell 3302, the present invention is adapted for tuningcells with any number of inductors 3810 and corresponding switches 3830,including greater numbers of inductors and switches. Furthermore, othercircuit component types than inductors can be used as phase-adjustmentelements for cells 3302, including capacitors.

According to embodiments of the present invention, the function ofswitches 3830 is provided by nanowire-based devices. Such nanowire-baseddevices include diode or field effect transistor (FET) types. In theembodiments described herein, each of cells 3302 incorporate one fullset of adjustment controls to maximize tunability and phase-controlflexibility. Note that in alternative embodiments, not every cell 3302requires phase control. Thus in some embodiments, every other cell 3302,or other multiples of cells 3302, will include phase adjustment control.

In embodiments, cells 3302 are approximately 5–10 mm in length/width,although they can have other sizes. Thus, it is impractical to mountstandard packaged commercial devices to cells 3302. For example, FIG. 41shows a scale drawing where three conventional inductors 4102 a–4102 cand corresponding switches 4104 a–4104 c are mounted to a cell 3302. Thecomponents used are commercial-off-the-shelf (COTS) technology.Inductors 4102 a–4102 c are shown as commercially available 0603inductors, and switches 4104 a–4104 c are shown as commerciallyavailable SOT-23 FETs. Note that because of the sizes of the componentspresent in the COTS example of FIG. 41, little free area exists on cell3302. Furthermore, additional circuit components required for RFbypassing, filtering, and bias control, are not present on cell 3302.Thus, a practical three-switched inductor implementation usingcommercially available components is difficult to implement.

FIG. 42 shows a scale drawing of a cell 3302 that mounts ananowire-based phase-adjustment circuit 4200, according to an embodimentof the present invention. In FIG. 42, phase-adjustment circuit 4200includes phase adjustment elements 4202 a–4202 c and nanowire-basedtransistors 4204 a–4204 c. In the example of FIG. 42, phase adjustmentelements 4202 a–4202 c are shown as microstrip inductors. In FIG. 42, asignificant amount of free space is available on cell 3302. Space existson cell 3302 in FIG. 42 to add additional components required for apractical design. Thus, as shown by a comparison of FIGS. 41 and 42, theuse of nanowire-based transistors 4204 enable the implementation ofpractical tunable cells 3302.

FIG. 43 shows an expanded view showing further detail of examplenanowire-based transistor 4204 a, according to an embodiment of thepresent invention. Nanowire-based transistor 4204 a is formed by a filmof nanowires, in a similar fashion as described above for semiconductordevice 1100 shown in FIG. 11, or other nanowire-based transistorsdescribed elsewhere herein. For example, as shown in FIG. 43,nanowire-based transistor 4204 a includes a drain contact 4302 a, a gatecontact 4206 a, a source contact 4304 a, and a thin film of nanowires4320. Thin film of nanowires 4320 can be formed or patterned asdescribed above for thin film of nanowires 1000 shown in FIG. 10, or asdescribed elsewhere herein. The nanowires of thin film of nanowires 4320can be aligned or non-aligned, can include a polymer if required, andcan include any other variation described herein. Thin film of nanowires4320 operates as a “channel” for nanowire-based transistor 4204 a, andcan be P-doped, N-doped, or a combination of P- and N-doping, dependingon the desired characteristics for nanowire-based transistor 4204 a.

As shown in FIG. 43, thin film of nanowires 4320 has a length greaterthan a length 4310. Length 4310 is substantially equal to a distancebetween drain contact 4302 a and source contact 4304 a. Thus, the lengthof thin film of nanowires 4320 is enough for thin film of nanowires 4320to adhere to, and make sufficient electrical contact with drain contact4302 a and source contact 4304 a for nanowire-based transistor 4204 a tooperate. Nanowires of thin film of nanowires 4320 may be formed orselected to have a length of length 4310 or greater, or may have otherlengths.

As shown in FIG. 42, the relatively small sizes of nanowire-basedtransistors 4204 leaves a large area on cell 3302 for placement ofphase-adjustment elements 4202, such as the microstrip inductors shown,and for other required components. Thus, greater numbers ofphase-adjustments elements may be mounted on cell 3302 to providegreater degrees of phase adjustment, and therefore, a greater degree ofbeam-steering control for beam-steering reflector 3200, whenincorporating nanowire-based transistors 4204.

Note that integrated circuit dies containing control electronics couldbe attached and wire-bonded to a cell 3302 instead of using the COTS ornanowire devices. However, conventional wire-bonding machines have beendesigned for the manipulation of wafer-sized objects, typically 3″–6″(7.6–15.2 cm) diameter wafers. Thus, expensive retrofits to thesemachines would be required to build active FSSs of any size andcomplexity. As such, the use of nanowire-based high-performancemacroelectronic substrates for cells 3302 is critical for practicalimplementations of beam-steering arrays.

Various processes can be used to integrate phase adjustment elements4202 and nanowire-based transistors 4204 of nanowire-basedphase-adjustment circuit 4200 in cells 3302. For illustrative purposes,example processes are described below for integrating nanowire-basedphase-adjustment circuits with cells 3302. However, the presentinvention is not limited to these examples.

FIG. 44 shows an example embodiment for formation of a beam-steeringreflector 700, where a PMC structure 4402 is formed separately fromnanowire-based phase adjustment circuits 4410 a–4410 d, and subsequentlycombined. Note that PMC structure 4402 can be fabricated usingconventional manufacturing techniques. Nanowire-based phase adjustmentcircuits 4410 a–4410 d are formed on a substrate 4404. Substrate 4404can be formed from any substrate material, including KAPTON. Openings(not shown in FIG. 44) are formed in substrate 4404 to provide access toends 3804 a and 3804 b of vias 3802 a and 3802 b for nanowire-basedphase adjustment circuits 4410 a–4410 d. Substrate 4404 is subsequentlyattached to PMC structure 4402, using an adhesive material, such as anepoxy or laminate material.

Note that in an alternative embodiment, nanowire-based phase adjustmentcircuits can be formed directly on a PMC structure. For example, FIG. 45shows an example where nanowire-based phase adjustment circuits areformed on PMC structure 4402. As shown in FIG. 45, an electricallyinsulating/dielectric material layer 4502 is applied to secondelectrically conductive layer 3404 of PMC structure 4402. Nanowire-basedtransistors can then be formed directly on dielectric material layer4502. In the example of FIG. 45, two nanowire-based transistors are inthe process of formation. As shown in FIG. 45, after applying dielectricmaterial layer 4502 to PMC structure 4402, dielectric material layer4502 is metallized with first and second sets of contacts 4520 a and4520 b, and with vias. Phase-adjustment elements can also be applied.For example, inductors 4530 a and 4530 b are shown formed on dielectricmaterial layer 4502 in FIG. 45. Inductors 4530 a and 4530 b can becreated on dielectric material layer 4502 using microstrip or striplinetechniques, as well as by applying wirewound type inductor devices.

Subsequently, as shown in FIG. 46 for a single nanowire-based transistor4204, a thin film of nanowires 4602 is applied to dielectric materiallayer 4502 and bonded to contacts 4520. As shown in the example of FIG.46, nanowires 4604 of thin film of nanowires 4602 are coated with adielectric material 4610 to create a gate dielectric. In this manner, aplurality of nanowire-based transistors 4204 can be created. A via 4620through dielectric material layer 4502 is shown in FIG. 46 thatelectrically couples a source electrode of nanowire-based transistor4204 to second electrically conductive layer 3404 of PMC structure 4402,which typically operates as a ground or other reference potential plane.

In another embodiment for forming a beam-steering reflector 3200, thinfilms of nanowires can be applied to a surface, such as substrate 4404.Gate, source, and drain contacts are patterned on the thin films ofnanowires. A dielectric material, such as dielectric material layer4502, is then formed on the substrate, covering the thin films ofnanowires and contacts. For example, the dielectric material can be alow-T_(p) dielectric. Vias are drilled and conductively filled. A PMCstructure can then be formed on the dielectric material. Thus, in thisembodiment, the resulting thin films of nanowires are on the samesurface of the substrate as the PMC structure.

Numerous other processes for forming beam-steering reflector 3200 willbe apparent to persons skilled in the relevant art(s) from the teachingsherein. Alternative embodiments for nanowire-based phase adjustmentcircuits are described in the following subsection.

Multi-Antenna Element Beam-steering Array Embodiments

This subsection describes example beam-steering arrays that incorporatea plurality of individual antenna elements. Although the followingdescription shows particular antenna configurations for illustrativepurposes, it is to be understood that the nanowire technology describedherein is applicable to any type of antenna incorporating a plurality ofindividual antenna elements, and such antennas are within the scope andspirit of the present invention.

FIG. 47 shows an example beam-steering array 4700, according to anexample embodiment of the present invention. Beam-steering array 4700shown in FIG. 47 is a multiple antenna element variation ofbeam-steering array 608, shown in FIGS. 6A and 6B. As shown in FIG. 47,beam-steering array 4700 includes a plurality of antenna elements 4702a–4702 n. Beam-steering array 4700 can include any number of antennaelements 4702, as required by a particular application. In theembodiment shown in FIG. 47, antenna elements 4702 a–4702 n are formedon a common substrate 4704. Substrate 4704 can be any substrate typesuitable for mounting antenna elements, such as the substrates describedelsewhere herein, or otherwise known. In alternative embodiments,antenna elements 4702 a–4702 n can be formed on more than one substrate,including each of antenna elements 4702 a–4702 n being formed on aseparate substrate. In further embodiments, antenna elements 4702 a–4702n may not be required to be formed on substrates at all.

Antenna elements 4702 a–4702 n each include a respective one of antennas4706 a–4706 n, one of adjustable phase shifters 4708 a–4708 n, and oneof antenna input/output lines 4710 a–4710 n. For each antenna element4702, antenna 4706 receives and transmits a signal. Antenna input/outputline 4710 conducts a signal to be transmitted to antenna 4706, andconducts a received signal from antenna 4706. Adjustable phase shifter4708 adjusts a phase shift of the signals conducted through antennainput/output line 4710 to and from antenna 4706.

Each of antenna elements 4702 a–4702 n are tunable. Thus, antennaelements 4702 a–4702 n can be collectively used as a beam-steeringarray, as described above for beam-steering array 3108. For example, byadjusting a phase of signals passing through antenna input/output lines4710 a–4710 n with adjustable phase shifters 4708 a–4708 n, a cumulativesignal transmitted by antenna elements 4702 a–4702 n can be re-directed,including selecting a particular direction, focusing, and spreading thetransmitted signal. Furthermore, adjustable phase shifters 4708 a–4708 ncan be used to receive signals from particular directions.

Note that in an embodiment, antenna input/output lines 4710 a–4710 nsupply the same signal to each of their respective antenna elements 4702a–4702 n. In alternative embodiment, one or more of antenna input/outputlines 4710 a–4710 n supply different signals to the respective antennaelements 4702 a–4702 n. For example, in such an embodiment, one or moreof antenna input/output lines 4710 a–4710 n may supply signalsphase-shifted by different amounts for transmission, so that some or allof antenna elements 4702 a–4702 n may not require adjustable phaseshifters 4708 a–4708 n, and may instead have no phase shift, or have“hard-wired” phase shifts. The “hard-wired” phase shifts may beimplemented using nanowire-enabled transistors, or othernanowire-enabled elements. A reflector-type beam-steering array asdescribed above may be similarly configured.

Embodiments for adjustable phase shifter 4708 are described in thefollowing subsection. These embodiments are provided for illustrativepurposes, and are not limiting.

Adjustable Phase Shifter Embodiments

Embodiments for an adjustable or variable phase shifter that can be usedas adjustable phase shifters 4708 a–4708 n shown in FIG. 47 aredescribed in this subsection. The adjustable phase shifter can be usedother applications than described above, including any variety ofapplications that require adjustable phase delays for an electricalsignal. The adjustable phase shifter can be used to provide any numberof phase delays, nearly approximating a continuous adjustable phasedelay in some embodiments. When applied to a beam-steering array, anadjustable phase shifter can be used to provide a very fine degree ofdirectional control for an antenna beam.

Electronically controllable variable phase shift networks are useful formany applications. At radio frequencies (e.g., above approximately 300kHz), phase shift networks can be used in antenna beam-steering arrays,such as described above, so that each antenna element either radiates orreflects radiation with a controllable phase shift. By suitablyadjusting the phase of each element of an array of antenna elements orreflectors, the antenna radiation pattern can be modified. As describedabove, this can be used to scan for a transmission power maximum of anantenna more rapidly than can be accomplished mechanically. This canalso be used to point an antenna toward a minima (or null) direction, toeliminate an interfering signal.

Preferably, such phase shift networks are small, inexpensive, low loss,and adjustable in small increments. For some antenna designs, it isadvantageous to be able to apply the phase shifter to a flexible fabricat a low temperature. Conventionally, solid state devices such as FETs,PIN diodes, and/or varactor diodes are used in adjustable phaseshifters. However, these devices are fabricated on rigid semiconductorsubstrates. The devices are coupled to each other and to the antennausing solder or wire bonding. Furthermore, such devices frequentlyrequire additional circuit components for biasing purposes. Thesedisadvantageous attributes increase cost, require elevated temperaturesduring processing, and limit the substrate types to which the devicescan be attached. Amorphous transistors could possibly overcome some ofthese difficulties, but their performance at RF and microwavefrequencies is inadequate.

The present invention discloses an adjustable phase shifter thatovercomes these limitations. FIG. 48 shows a flowchart 4800 providingexample steps for forming an adjustable phase shifter on a substrate,according to embodiments of the present invention. The steps of FIG. 48do not necessarily have to occur in the order shown, as will be apparentto persons skilled in the relevant art(s) based on the teachings herein.Other structural embodiments will be apparent to persons skilled in therelevant art(s) based on the following discussion. The steps offlowchart 4800 are described in detail below.

Flowchart 4800 begins with step 4802. In step 4802, a conductor line isformed on the substrate, wherein the conductor line includes a firstconductive segment and a second conductive segment. For example, FIG. 49shows a conductor line 4902 formed on a substrate 4900. Conductor line4902 can be any type of electrical conductor, including a metal trace ortransmission line. For example, conductor line 4902 can be formed over aground plane in a transmission line structure. Conductor line 4902 canalternatively be a microstrip, stripline, coplanar waveguide, or otherconductor type. Furthermore, substrate 4900 can be any type of substrateon which conductors for electrical signals can be formed, including anytype of substrate described elsewhere herein, or otherwise known.Conductor line 4902 can be formed on substrate 4900 using anyconventional process.

As shown in FIG. 49, conductor line 4902 includes a first conductivesegment 4904 and a second conductive segment 4906. A third conductivesegment 4908 is also included in conductor line 4902, in series betweenfirst and second conductive segments 4904 and 4906. In an embodiment,first and second conductive segment 4904 and 4906 are coplanartransmission line-like strips, forming a ¼ wavelength long transmissionline. Third conductive segment 4908 effectively forms an electricalshort at an end of the transmission line. Such a ¼ wavelengthtransmission line, when shorted at one end, electrically appears as aninductor at the other end. By changing a length of the formedtransmission line, as described below, the apparent inductor changesvalue. Thus, conductor line 4902, configured in this manner, can be usedto change a phase delay of a signal, such as a signal coupled to one ofantennas 4706 a–4706 n, shown in FIG. 47.

In step 4804, a thin film of nanowires is formed on the substrate inelectrical contact with the first conductive segment and the secondconductive segment. For example, FIG. 50 shows a thin film of nanowires5002 formed on substrate 4900, according to an embodiment of the presentinvention. Thin film of nanowires 5002 is similar to thin film ofnanowires 1000 shown in FIG. 10, for example. Thin film of nanowires5002 can be patterned or formed, and can include any of the variationsdescribed herein for nanowires. As shown in FIG. 50, thin film ofnanowires 5002 is formed to be in electrical contact with both of firstand second conductive segments 4904 and 4906 of conductor line 4902.

In step 4806, a plurality of gate contacts are formed in electricalcontact with the thin film of nanowires. For example, FIG. 51 shows aplurality of gates contacts 5102 a–5102 n formed in electrical contactwith thin film of nanowires 5002, to form an adjustable phase shifter5100, according to an embodiment of the present invention. In thismanner, a plurality of nanowire-based transistors 5116 a–510 n areformed in thin film of nanowires 5002, each controlled by one of gatecontacts 5102 a–5102 n. First and second conductive segments 4904 and4906 function as common source and drain contacts for nanowire-basedtransistors 5110 a–5110 n.

Gate contacts 5102 a–5102 n can be formed on substrate 4900 prior toapplying thin film of nanowires 5002 to substrate 4900 in step 4804, orcan be formed on thin film of nanowires 5002 after step 4804. Gatecontacts 5102 a–5102 n can be any contact type, including a conductivepolymer, metal, polysilicon, or other contact type describe herein orotherwise known. Any number of gate contacts 5102 can be formed,depending on the degree of phase adjustment control desired by theparticular application. The greater the number of gate contacts 5102that are present, the greater the degree of phase control. Furthermore,a spacing of gate contacts 5102 a–5102 n can be uniform or non-uniform.For example, the spacing of gate contacts 5102 a–5102 n can bedistributed in a binary weighted fashion.

Control signals are coupled to each of gate contacts 5102. Differentdegrees of phase shift are provided by activating various gate contacts5102 a–5102 n with the control signals. Activation of each of gatecontacts 5102 a–5102 n causes the respective nanowire-based transistor5110 to electrically short or bypass different portions of conductorline 4902. This provides different length electrical paths throughconductor line 4902, thus providing different phase delays.

In this manner, adjustable phase shifter 5100 allows a phase of anelectrical signal transmitted through conductor line 4902 to be adjustedby changing a voltage applied to at least one gate contact of theplurality of gate contacts 5102 a–5102 n. In an antenna application,such as described above, adjustable phase shifter 5100 provides avariable inductance, therefore adjusting a reflected phase of a givenantenna element of an array.

Note that conductor line 4902, gate contacts 5102, and thin film ofnanowires 5002 can be formed on substrate 4900 in any order.

The nanowires of thin film of nanowires 5002 can be aligned ornon-aligned. For example, FIG. 52A shows an example adjustable phaseshifter 5100, having a thin film of nanowires 5002 with alignednanowires, according to an embodiment of the present invention. FIG. 52Bshows a cross-sectional view of adjustable phase shifter 5100 of FIG.52A. The nanowires are shown aligned parallel to an axis 5202 betweenfirst and second conductive segments 4904 and 4906. Furthermore, in theembodiment shown in FIG. 52A, the nanowires of thin film of nanowires5002 have a length approximately equal to a distance between first andsecond conductive segments 4904 and 4906, although in other embodiments,the nanowires can have other lengths.

Note also in FIGS. 52A and 52B, not every nanowire of thin film ofnanowires 5002 is incorporated into a nanowire-based transistor 5110. Inother words, not every nanowire must be in electrical contact with agate contact 5102. For example, a nanowire 5250 shown in FIGS. 52A and52B is such a nanowire. Nanowires of thin film of nanowires 5002 can beformed to be non-electrically conductive in at least one direction(i.e., are in an inversion mode), so that if some nanowires are notincluded in any nanowire-based transistor 5110, they do not affectoperation of adjustable phase shifter 5100.

In an embodiment, substrate 4900 can be configured as shown in FIG. 52B.As shown in FIG. 52B, substrate 4900 includes a dielectric materiallayer 5260 attached to an electrically conductive layer 5270, which canfunction as a ground or other potential plane.

Note that in embodiments, multiple sections of nanowire films can beformed on substrate 4902 to form nanowire-based transistors 5110. Forexample, FIG. 53 shows a plurality of thin films of nanowires 5002a–5002 n formed on substrate 4902, according to an embodiment of thepresent invention. Each of the plurality of thin films of nanowires 5002a–5002 n is activated by a respective one of gate contacts 5102 a–5102n.

In embodiments, the amount of phase delay provided by a particularnanowire-based transistor 5110 is dictated by the change in length ofthe electrical path through conductor line 4902 provided by thenanowire-based transistor 5110. In further embodiments, loads can beapplied to conductor line 4902 to provide additional or controlled phasedelay amounts. For example, circuit components such as inductors,capacitors, and resistors can be used to provide altered phase delays.

FIG. 29 shows conductor line 4902 (substrate 4900 not shown) withincorporated loads to provide phase delay, according to an exampleembodiment of the present invention. As shown in FIG. 54, conductor line4902 has first and second inductors 5402 a and 5402 b (shown as genericinductor elements) formed therein to provide phase delays. Furthermore,conductor line 4902 has first, second, and third capacitors 5404 a–5404c (shown as generic capacitor elements) coupled thereto to providealtered phase delays. FIG. 55 shows conductor line 4902 of FIG. 54, withthin films of nanowires 5002 a–5002 c formed thereon to form a pluralityof nanowire-based transistors 5110 a–5110 c. A phase delay provided byactivating any one or none of transistors 5110 a–5110 c is affected bycombinations of inductors 5402 a and 5402 b and capacitors 5404 a–5404c. In embodiments, any number of inductors, capacitors, and/or otherload components can be incorporated into conductor line 4902 to providephase delays, as needed.

As described above, nanowires of thin film of nanowires 5002 can becoated with an insulating material that functions as a gate dielectric.In an embodiment, this insulating material can be removed from thenanowires in the source and drain regions of first and second conductivesegments 4904 and 4906 to provide improved electrical contact.

Note that the nanowire-based transistors can be used as either high orlow impedance switches, depending on the voltage applied to gatecontacts 5102. Note that most of the parasitics (capacitance to ground,capacitance to gate, etc.) are absorbed into conductor line 4902, andthus do not adversely effect the adjustable phase shifter circuit. Thenanowires can be N- or P-doped, and can be configured as either anenhancement or depletion mode transistor type.

Furthermore note that nanowire-based PIN diodes and nanowire-basedvaractors may be alternatively used instead of the nanowire-basedswitches described above, as would be understood by persons skilled inthe relevant art(s).

Active Acoustic Cancellation Embodiments

This subsection describes example arrays of actuators used tosubstantially reduce or cancel acoustic signals, such as audible noise.According to embodiments of the present invention, thin films ofnanowires or nanotubes are used to enable such acoustic cancellationover large areas. Although the following description shows particularacoustic cancellation configurations for illustrative purposes, it is tobe understood that the nanowire technology described herein isapplicable to other types of acoustic cancellation devices, and suchacoustic cancellation devices are within the scope and spirit of thepresent invention.

Some conventional headphones incorporate technology that monitors noisearound the headphones, and transmits a pattern of acoustic waves in anattempt to substantially cancel the outside noise. The transmittedpattern of acoustic waves is transmitted with an opposite phase to thatof the noise. This transmitted pattern attempts to silence the noise,making it easier to hear what is being played through the headphones.One company that manufactures such headphones is Bose Corporation, ofFramington, Mass.

This technology works by receiving incoming acoustic signals, such asnoise, processing the incoming signal, and calculating a complementaryfrequency that is out of phase with the incoming signal. The calculatedfrequency is sent to an actuator, which transmits a canceling acousticsignal, causing at least partial acoustic cancellation. This technology,however, is limited to small area applications, such as the headphones.

According to the present invention, it is possible to provide acousticcancellation to a very large area by creating an array of actuators,each having a corresponding receiver and processor to calculate acancellation response from the position of the actuator. Such a largearea active acoustic cancellation system has many useful applications.For example, large area active acoustic cancellation can be used topartially or completely cancel the sound emanating from objects such asa car, a bus, or even an airplane. In example military applications,sounds from objects such as a tank or submarine can be partially orcompletely cancelled.

Using conventional technology, it is impractical to produce thenecessary electronics over a large area to support an array ofactuators. Each required processor is typically a high-performanceelectronic device formed from a silicon wafer. The required electronicsmust further supply enough power to function at acoustic frequencies andprovide substantial gain to drive the actuators.

The nanowire-enabled macroelectronic materials of the present inventiondescribed herein allow the formation of a large-area circuit capable ofdriving an array of actuators over a large area. Furthermore, the arrayis lightweight, conformal, and can be applied over any structure. Stillfurther, by incorporating piezoelectric nanowires into a macroelectronicfilm, it is also possible to incorporate the actuators into a singlesubstrate with the electronics to form a truly macroelectronicintegrated active acoustic cancellation system. This actuators andelectronics can be applied to a flexible substrate, and the flexiblesubstrate can be attached to an object. Alternatively, the actuators andelectronics can be applied directly to the surface of the object.

FIG. 56 shows an array 5600 of cells 5602 a–5602 n. Each of cells 5602a–5602 n includes an actuator and related electronics used for activeacoustic cancellation, according to an embodiment of the presentinvention. Embodiments of array 5600 can include any number of cells5602, as required by the particular application. For example, greaternumbers of cells 5602 can be used to cancel noise or other sounds overlarger sized objects.

FIG. 32 shows an example implementation of array 5600 of FIG. 56, beingused to monitor and cancel incoming sounds, according to an embodimentof the present invention. As shown in FIG. 32, an input sound wave 3200encounters array 5600. Input sound wave 3200 includes waves of sound,including noise. Input sound wave 3200 include a plurality of inputsound wave elements 3202 a–3202 d, which are the portions of input soundwave 3200 received at corresponding ones of cells 5602 a–5602 n of array5600. Each of cells 5602 a–5602 d receives and processes thecorresponding one of sound wave elements 3202 a–3202 d. Each of cells5602 a–5602 d generate and transmit a cancellation sound signal 3204a–3204 c, accordingly. Each of cancellation sound signals 3204 a–3204 dare generated to have substantially the same frequency and oppositephase of the corresponding one of received sound wave elements 3202a–3202 d, to substantially cancel the corresponding one of receivedsound wave elements 3202 a–3202 d. In this manner, input sound wave 3200is reduced or eliminated.

FIG. 58 shows an example detailed block diagram of a cell 5602 of FIG.56, according to an example embodiment of the present invention. Cell5602 includes an antenna 5802, a receiver 5804, a processor 5806, anactuator interface circuit 5808, and an actuator 5810. Alternativeconfigurations for cell 5602, including additional or alternativecomponents than shown in FIG. 58, are applicable to the presentinvention.

Antenna 5802 receives a first acoustic signal, which can be a sound waveelement 3202, for example. Antenna 5802 is any element that can receivesound or audio signals, and produce an electrical signal representativeof the received first acoustic signal.

Receiver 5804 is coupled to antenna 5802. Receiver 5804 receives theelectrical signal output by antenna 5802, and generates a signalappropriate for processing by processor 5806. For example, in anembodiment, receiver 5804 may include an analog-to-digital converter todigitize an analog electrical signal output by antenna 5802. Receiver5804 outputs a receiver output signal 5812.

Processor 5806 is coupled to receiver 5804. Processor 5806 receives andprocesses receiver output signal 5812. Processor 5806 determinesinformation regarding signal 5812. For example, in an embodiment,processor 5806 can determine a frequency present in signal 5812 and anamplitude of signal 5812. Processor 5806 also determines a phase ofsignal 5812. Processor 5806 then uses the determined information todetermine an amplitude, a frequency, and a phase of a desiredcancellation signal. Processor 5806 outputs a control signal 5814 thatcorresponds to an amplitude, a frequency, and a phase of the desiredcancellation signal. Processor 5806 can include any hardware, software,firmware, or any combination thereof necessary to perform its functions,including an integrated circuit processor and/or digital logic.

Actuator interface circuit 5808 is coupled to processor 5806. Actuatorinterface circuit 5808 conditions control signal 5814 prior to beinginput by actuator 5810. For example, actuator interface circuit 5808 caninclude a digital-to-analog converter to convert signal 5814 to analog,when signal 5814 is digital. Furthermore, actuator interface circuit5808 can include one or more amplifiers to provide required gain andcurrent for actuator 5810. Actuator interface circuit 5808 can alsoinclude switches/relays that gate current flow to actuator 5810. Asdescribed below, in an embodiment, actuator interface circuit 5808 caninclude one or more nanowire-enabled transistors to control current flowto actuator 5810. Actuator interface circuit 5808 outputs actuator inputsignal 5816. Actuator interface circuit 5808 can include any hardware,software, firmware, or any combination thereof necessary to perform itsfunctions.

Actuator 5810 receives actuator input signal 5816, and outputs a secondacoustic signal, which can be cancellation sound signal 3204, forexample. Actuator 5810 is any element or elements that can convert anelectrical signal to a sound or audio signal, including one or moreaudio speakers or piezoelectric devices. As described below, in anembodiment, actuator 5810 can include piezoelectric nanowires that areused to generate an acoustic, sound, or audio output signal.

FIG. 59 shows an example 2×2 array 3100 formed on a substrate 5900, witheach of cells 3102 a–3102 d configured as shown in FIG. 58, according toan example embodiment of the present invention. In the configuration ofFIG. 59, it is difficult to include the electronics and actuators ofeach of cells 3102 a–3102 d on a single substrate 5900. FIGS. 59 and 60show example embodiments for array 3100, which include nanowire films.The inclusion of nanowire films enables the formation of array 3100 on asingle substrate 5900. These embodiments are further described below.

FIG. 60 shows array 3100 of FIG. 59, with each cell 3102 incorporating ananowire-based actuator interface circuit 6008, according to an exampleembodiment of the present invention. Incorporating nanowires innanowire-based actuator interface circuit 6008 allows for higher levelsof performance, reduced space requirements, flexibility, and additionalbenefits further described elsewhere herein. Example embodiments fornanowire-based actuator interface circuit 6008 are provided below.

FIG. 61 shows array 3100 of FIG. 60, where each cell 3102 incorporates ananowire-based actuator 6110, according to an example embodiment of thepresent invention. For example, piezoelectric nanowires are incorporatedin nanowire-based actuator 6110 to generate cancellation sound signal3204. In an embodiment, one or more thin films of piezoelectricnanowires are present in nanowire-based actuator 6110. Each thin film ofpiezoelectric nanowires is configured to generate a respectivefrequency, so that a plurality of output frequencies can be present incancellation sound signal 3204. Alternatively, variations in a currentand/or voltage applied to a thin film of piezoelectric nanowires inactuator 6110 can be used to generate different frequencies.Furthermore, incorporating nanowires in nanowire-based actuator 6110allows for high levels of performance, reduced space requirements,flexibility, and additional benefits described elsewhere herein. Exampleembodiments for nanowire-based actuator 6110 are provided below.

FIGS. 62 and 63 show example embodiments for nanowire-based interfacecircuit 6008 and nanowire-based actuator 6110, according to the presentinvention.

In the example embodiment of FIG. 62, nanowire-based actuator interfacecircuit 6008 includes a nanowire-based transistor 6202 (other componentsof nanowire-based actuator interface circuit 6008 not shown).Nanowire-based transistor 6202 includes a source contact 6204, a draincontact 6206, a gate contact 6208, and a thin film of nanowires 6210.Nanowires of thin film of nanowires 6210 can be coated with a dielectricmaterial to create a gate dielectric. Gate contact 6208 receives acontrol signal that causes thin film of nanowires 6210, which operatesas a channel for nanowire-based transistor 6202, to operatenanowire-based transistor 6202.

In FIG. 62, nanowire-based actuator 6110 includes a thin film ofpiezoelectric nanowires 6214. When nanowire-based transistor 6202 is“on”, thin film of piezoelectric nanowires 6214 conducts a currentbetween drain contact 6206 and a contact 6212. When thin film ofpiezoelectric nanowires 6214 is conducting current, thin film ofpiezoelectric nanowires 6214 produces a frequency transmitted incancellation sound signal 3204.

FIG. 63 shows an embodiment where nanowire-based actuator interfacecircuit 6008 and nanowire-based actuator 6110 are overlapping.Nanowire-based actuator interface circuit 6008 and nanowire-basedactuator 6110 include a nanowire-based transistor 6302. Nanowire-basedtransistor 6302 includes a source contact 6304, a drain contact 6306, agate contact 6312, and a thin film of piezoelectric nanowires 6308.Nanowires of thin film of piezoelectric nanowires 6308 can be coatedwith a dielectric material to create a gate dielectric. Gate contact6312 receives a signal that causes thin film of piezoelectric nanowires6308, which operates as a channel for nanowire-based transistor 6302, tooperate nanowire-based transistor 6302. When nanowire-based transistor6302 is “on”, thin film of piezoelectric nanowires 6308 conducts acurrent between source contact 6304 and drain contact 6306. When thinfilm of piezoelectric nanowires 6308 is conducting current, thin film ofpiezoelectric nanowires 6308 produces a frequency present incancellation sound signal 3204.

The embodiments described above for nanowire-based actuator interfacecircuit 6008 and nanowire-based actuator 6110 are provided forillustrative purposes. The present invention is applicable to furtherembodiments for nanowire-based actuator interface circuit 6008 andnanowire-based actuator 6110, as would be understood by persons skilledin the relevant art(s) from the teachings herein.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the presentinvention. Thus, the breadth and scope of the present invention shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

1. A radio frequency identification (RFID) tag, comprising: an antenna;a beam-steering array that includes a plurality of tunable elements,each tunable element including: a plurality of phase-adjustmentcomponents; a switch corresponding to each phase-adjustment component,said switch including a transistor formed by a thin film of nanowires inelectrical contact with source and drain contacts; wherein said switchenables said corresponding phase-adjustment component to change a phaseof said tunable element; wherein an electromagnetic (EM) signaltransmitted by said antenna is redirected by said beam-steering array.2. The RFID tag of claim 1, wherein said beam-steering array focusessaid EM signal.
 3. The RFID tag of claim 1, wherein said eachphase-adjustment element comprises an inductor.
 4. The RFID tag of claim3, wherein said inductor is a micro-strip inductor.
 5. The RFID tag ofclaim 1, wherein said each phase-adjustment element comprises acapacitor.
 6. The RFID tag of claim 1, wherein said nanowires arealigned substantially parallel to their long axis.
 7. The RFID tag ofclaim 1, wherein said nanowires are randomly aligned.
 8. The RFID tag ofclaim 1, wherein said nanowires are coated with a dielectric material tothereby form a gate dielectric.
 9. The RFID tag of claim 1, wherein saidnanowires have doped cores.
 10. The RFID tag of claim 1, wherein saidnanowires have doped shells.
 11. The RFID tag of claim 1, wherein saidnanowires have doped cores and shells.
 12. The RFID tag of claim 1,wherein said nanowires are N-doped.
 13. The RFID tag of claim 1, whereinsaid nanowires are P-doped.
 14. The RFID tag of claim 1, wherein saidbeam-steering array is a beam-steering reflector, wherein said tunableelements are tunable cells that are co-planar.
 15. The RFID tag of claim14, wherein each tunable cell comprises a resonant structure.
 16. TheRFID tag of claim 15, wherein said switch enables the electricalcoupling of said corresponding phase adjustment component to saidresonant structure to change a phase of said tunable cell.
 17. The RFIDtag of claim 16, wherein each said resonant structure comprises: a firstelectrically conductive layer; a second electrically conductive layer; adielectric layer between said first and said second electricallyconductive layers; and an electrically conductive via through saiddielectric layer having a first end coupled to said first electricallyconductive layer and having a second end extending through an opening insaid second electrically conductive layer.
 18. The RFID tag of claim 17,wherein said nanowire film-based transistor is attached to said secondelectrically conductive layer, wherein a terminal of said nanowirefilm-based transistor is coupled to said second end of said electricallyconductive via.
 19. A radio frequency identification (RFID) tag,comprising: a beam-steering array that includes a plurality of tunableantenna elements, each tunable antenna element including: a plurality ofphase-adjustment components; a switch corresponding to eachphase-adjustment component, said switch including a transistor formed bya thin film of nanowires in electrical contact with source and draincontacts; wherein said switch enables said correspondingphase-adjustment component to change a phase of said tunable antennaelement; wherein an electromagnetic (EM) signal transmitted by saidbeam-steering array is directed by controlling the phase of each of saidplurality of tunable antenna elements.
 20. The RFID tag of claim 19,wherein said tunable elements are tunable transmission line segments.21. The RFID tag of claim 20, wherein said switch shorts saidtransmission line segment to change a length of said transmission linesegment to change a phase of said transmission line segment.
 22. Amethod for steering an electromagnetic (EM) signal related to a radiofrequency identification (RFID) tag, comprising: (a) receiving the EMsignal at a beam-steering array of the RFID tag, wherein thebeam-steering array includes a plurality of tunable elements; and (b)adjusting a phase of a tunable element of the beam-steering array tore-direct the EM signal, including the step of: (1) actuating a switchcorresponding to a phase-adjustment component coupled to the tunableelement to change a phase of the tunable element, the switch including atransistor formed by a thin film of nanowires in electrical contact withsource and drain contacts.
 23. The method of claim 22, furthercomprising: (c) prior to step (a), transmitting the EM signal from anantenna of the RFID tag; wherein step (a) comprises receiving the EMsignal from the antenna.
 24. The method of claim 23, wherein step (b)comprises: re-directing the EM signal towards a reader.
 25. The methodof claim 23, wherein step (a) comprises: receiving the EM signal from areader.
 26. The method of claim 25, further comprising: (c) scanning thebeam-steering array to determine a direction from which the EM signal isreceived.
 27. The method of claim 26, wherein step (c) comprises thesteps of: (1) performing step (b) for at least one tunable element ofthe beam-steering array; (2) measuring an amplitude of the received EMsignal; (3) comparing the measured amplitude with a previously measuredamplitude; (4) repeating steps (1)–(3) until a maximum measuredamplitude is determined.
 28. The method of claim 26, wherein step (b)comprises: re-directing the EM signal towards an antenna of the tag. 29.The method of claim 22, wherein step (b) comprises: focusing the EMsignal.
 30. The method of claim 22, wherein step (b) comprises:spreading the EM signal.
 31. The method of claim 22, wherein eachphase-adjustment element comprises an inductor, wherein said actuatingstep comprises: actuating the switch corresponding to the inductorcoupled to the tunable element to change a phase of the tunable element.32. The method of claim 22, wherein each phase-adjustment elementcomprises an capacitor, wherein said actuating step comprises: actuatingthe switch corresponding to the capacitor coupled to the tunable elementto change a phase of the tunable element.
 33. The method of claim 22,wherein the beam-steering array is a beam-steering reflector, whereinthe tunable elements are co-planar tunable cells comprising a resonantstructure, wherein step (1) comprises: actuating a switch correspondingto a phase-adjustment component coupled to the resonant structure tochange a phase of the resonant structure.
 34. A method for steering anelectromagnetic (EM) signal related to a radio frequency identification(RFID) tag, comprising: (a) transmitting the EM signal using abeam-steering array of the RFID tag, wherein the beam-steering arrayincludes a plurality of tunable antenna elements; and (b) adjusting aphase of a tunable antenna element of the beam-steering array tore-direct the EM signal, including the step of: (1) actuating a switchcorresponding to a phase-adjustment component coupled to the tunableantenna element to change a phase of the tunable element, the switchincluding a transistor formed by a thin film of nanowires in electricalcontact with source and drain contacts.
 35. The method of claim 34,wherein the tunable elements are tunable transmission line segments,wherein step (1) comprises: actuating a switch corresponding to aphase-adjustment component to short the transmission line segment tochange a length of the transmission line segment to change a phase ofthe transmission line segment.